Check out which companies are making headlines after the bell Wednesday:

Pershing Square Capital Management's William Ackman, told investors in a letter that it sold its Citigroup shares to help build a stake in Procter & Gamble . Ackman told CNBC his fund owns about $1.8 billion dollars worth of stock in P&G.(Click here for after-hours quotes.)

Microsoft - The software giant announced its new Windows 8 will be released on October 26. Shares edged higher in extended-hours trading. (Click here for extended-hours quote.)

IBM - The tech giant posted earnings of $3.51 a share, topping expectations for $3.42 a share. But the company posted revenue of $25.8 billion, missing Wall Street's expectations for $26.28 billion.Still, the company also raised its full-year earnings outlook , sending shares higher in after-hours trading. (Click here for extended-hours quote.)

American Express - Thecredit-card provider posted earnings of $1.15 a share, beating expectations for $1.09 a share. But the firm reported sales of $7.97 billion, below estimates for $8.1 billion. Shares edged lower in extended-hours trading. (Click here for extended-hours quote.)