Tens of thousands of junk Web pages, created only to lure search-engine users to advertisements, are proliferating like billboards strung along freeways. Now Microsoft researchers say they have traced the companies and techniques behind them.

The researchers uncovered a complex scheme in which a small group, creating false doorway pages, works with operators of Web-based computers who profit by redirecting traffic passed from search engines in one direction and then sending advertisements acquired from syndicators in the opposite direction.

"A small number of rogue actors who know what they are doing can create an enormous amount of disruption," said David L. Sifry, chief executive of Technorati, a blog-indexing company that works to keep junk pages of this sort out of its indexes. "It's sort of like putting a blindfold on you and spinning you around three times and then taking off the blindfold and showing you an ad."

Using questionable or illegal techniques to improve the ranking of a Web site in query results is known as search-engine spamming. The practice has proved to be a vexing problem for the major search companies, which struggle to prevent both spammers and companies specializing in improving legitimate clients' Web traffic - a field known as search-engine optimization - from undermining their page-ranking systems.