Web Ads Hidden Under Cloak of Invisibility

Wall Street journal reports that Kraft Foods, Greyhound Lines and Capital One Financial have bought some strange ads on the Internet lately. What's so strange about them is that they're invisible.

The companies might not have known about their invisible display ads—the kind that are supposed to appear alongside content on Web pages—if not for Ben Edelman, an assistant professor at Harvard Business School who studies Internet advertising.

Mr. Edelman says his research shows that all three marketers, and many others, have fallen victim to Web sites that use such ads as a way to sell more ad space than they have.