By Rolfe Winkler And Jack Marshall 

The war for advertising dollars between Google Inc. and Facebook Inc. may add a new front: email addresses.

Google is in talks to allow advertisers to target ads in search results at their existing customers, according to people familiar with the matter. Google has suggested that advertisers hand over customer information such as email addresses, which Google would then use to target the ads, the people said.

The move would mimic a popular advertising service launched in 2012 by Facebook called "custom audiences," which is helping the social network chip away at Google's lead in digital advertising. Facebook reported ad revenue grew 65% to $11.5 billion in 2014 while Google's advertising revenue grew 17% to $59.1 billion.

The proposed ads highlight the increasingly sophisticated ways that Internet companies track users' online and offline activities for marketing purposes. Such tactics might raise questions about how well customers understand what businesses do with personal information they volunteer to a website or through a store loyalty program.

The idea could also potentially appeal to any Web publisher that collects user data. Wall Street Journal-owner News Corp is exploring the possibility of offering advertisers similar functionality, according to people familiar with the matter. News Corp has suggested to marketers that they could target and track ads to their customers across the media company's properties using their email addresses.

Two people familiar with the matter said Google has talked about launching the new ads later this year or early next year. "We're always discussing potential product ideas with our clients," a Google spokesman said, "but have nothing new to share at this time."

Here's an example of how Google's ads would work, according to the people familiar with the matter: An office-supply retailer would give Google the email addresses of recent printer purchasers. The retailer could then bid to show ads when those people search for printer ink on google.com. The retailer could also tailor its Google ads based on other information it knows about the customer, including age, gender or prior purchases.

Google could also match customers using their Gmail addresses. Many users of Google services including Gmail, YouTube and Google Maps provide secondary email addresses and phone numbers that also could be used for matches.

For now, the ads would be limited to search results and wouldn't include banner ads, the people familiar with the matter said.

The ads would give marketers another way to reach customers when they are searching for products rather than providing more general ads when they aren't necessarily shopping. The data could also help both Google and advertisers better understand the relationship between online ads and consumers' real-world activities and purchases.

Facebook's similar "custom audiences" program helps advertisers target customers as they scroll through its Web page and mobile app. Facebook has detailed personal information for many of its 1.4 billion users, including their names, email addresses and phone numbers, which it matches against the customer information uploaded by advertisers.

Such ads help Facebook close the gap with Google in terms of Web users' intent. Google's $60 billion annual ad business is built around user searches, such as "flights to Charlotte" or "Chicago Cubs baseball hat." Facebook has personal information on its users, but they aren't necessarily flipping through their news feed looking to buy a flat-screen TV or to book a hotel room.

With custom audiences, Facebook can let a shoe retailer upload a list of people who bought running shoes two years ago and then target those people with ads because they may need another pair soon.

Facebook also allows advertisers to compile lists of "lookalikes"--that is, people that Facebook believes are similar to the advertisers' customers based on their age, gender or interests. Such ads are thought to be effective. Adobe Inc.'s Media Optimizer unit reported in one case study that lookalike ads led to triple the sales of regular Facebook ads.

Google is also discussing the possibility of creating "lookalike" audiences, according to one of the people familiar with the matter.

The company already allows advertisers to target search ads to users who have previously visited the advertiser's website. The proposed ads would be different because they would rely on information that customers have provided themselves, such as their email address.

Write to Rolfe Winkler at rolfe.winkler@wsj.com and Jack Marshall at Jack.Marshall@wsj.com

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