By David Benoit 

Yahoo Inc., in deciding to spin off its $40 billion stake in Alibaba Group Holding Ltd., addressed the biggest concern of investor Starboard Value LP: finding a tax-efficient way to monetize Yahoo's investment in the Chinese e-commerce giant.

But Yahoo's plan may not completely satisfy the activist shareholder, who had called for Yahoo to spin off its holdings in Alibaba and Yahoo Japan together into a separate entity from the main company, as well as other moves to cut costs and halt acquisitions.

To be sure, the Alibaba stake was the big fish, and investors widely cheered Yahoo's announcement Tuesday, sending the stock up around 7% to $51.43 in after-hours trading.

Starboard's suggestion that Yahoo merge its core business of Internet properties with rival AOL Corp. has gotten a lot of headlines, but the Alibaba stake was the activist's big concern. The stake makes up about $40 billion of Yahoo's $47 billion market capitalization, so every dollar saved on taxes on the separation of its Alibaba stake means significant more value to Yahoo shareholders.

RBC Capital Markets analyst Mark Mahaney earlier had estimated that for every five percentage points Yahoo saved on the tax bill for selling Alibaba, it would add $2 billion to its proceeds.

Those numbers meant this wasn't an issue just for Starboard. Yahoo executives said Tuesday they have spent two years working with experts to get this plan together.

Chief Financial Officer Kenneth Goldman said the company decided to not spin off Yahoo Japan at the same time because it could complicate the transaction.

"We are not saying we won't do something," Mr. Goldman said. "All we are saying is we want to get this done seamlessly and as cleanly as we possibly can."

Tax expert Bob Willens said Yahoo made the right move on Tuesday and that he didn't expect them to have included the Yahoo Japan stake in the spin.

On acquisitions, which Starboard wants Yahoo to stop, Chief Executive Marrisa Mayer defended her recent purchases and said she would be "disciplined" going forward.

And on expenses, the company said it continues to work on its costs, but didn't lay out specifics. The company's forecast for adjusted earnings before interest, taxes, depreciation and amortization implied profit margins would slide steeply from the year-earlier period.

While Starboard hasn't disclosed its full stake in Yahoo, only showing a small 0.8% position publicly, it has gained attention in recent months that can help earn it other shareholders' support.

Starboard is among the most prolific of activist investors and has made a splash over the past year as it found itself amid several high-profile deals. It is currently pushing the combination of office-supply companies Staples Inc. and Office Depot Inc., where it owns large stakes in both. And last year it won a closely watched fight to unseat the entire 12-person board of Darden Restaurants Inc., the parent of Olive Garden.

On Monday, it scored a victory in a campaign not quite in the spotlight, when paper-product producer MeadWestvaco Corp. agreed to merge with rival Rock-Tenn Co. in a deal that would create a $16 billion company. Starboard has a 6.1% stake in MeadWestvaco, whose stock rose 14% on the news Monday.

Write to David Benoit at david.benoit@wsj.com

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