Google, Yahoo and Microsoft will announce a common code of conduct relating to freedom of expression and the protection of privacy online.

Through a new organization, the Global Network Initiative, the companies will set out their principles for doing business in countries that restrict free speech online. Other participants in the initiative include human rights campaigners and socially responsible investors, said Brock Meeks, director of communications at the Center for Democracy and Technology, one of the organizations involved.

Google, Yahoo and Microsoft dominate the search engine market and also host blogs, Web mail services and forums that are used around the world, putting them in a powerful position to influence the discovery and discussion of information. They have been accused of abusing this position to aid censorship in some countries, by editing search-engine results or providing information about the real identities of critics of government policies.

The three companies will reveal the details of the new code of conduct in a conference call with news media at noon Eastern Time on Tuesday. For now, the domain globalnetworkinitiative.org registered by the Center for Democracy and Technology on Oct. 20 says simply "Coming soon."

Whether the code of conduct will make a significant difference to the companies' behavior remains to be seen: A Microsoft document created on Oct. 24, "Microsoft on the topic: Online freedom of expression" says the company will continue to comply with all local laws regarding censorship of search results and blog postings.

That censorship is something that earned it criticism from campaign group Human Rights Watch in a 2006 report entitled "Race to the Bottom:Corporate Complicity in Chinese Internet Censorship."

That report also criticized Skype for its policy of censoring sensitive words in online chats using instant messaging software developed with its Chinese partner Tom Online. Earlier this month, it became clear that the software was also storing details of the censored chats on a server in China, although Skype officials have denied knowledge of this.

Google, too, has proved itself keen to carry on censoring search results where governments ask it to. Last year, it recommended voting against a shareholder proposal that the company resist censorship efforts and notify users when it is required by governments to censor search results. The proposal was rejected.

Yahoo, meanwhile, has been criticized by human rights campaigners and by U.S. lawmakers for its role in the 2004 arrest of Chinese journalist Shi Tao, who used the Yahoo e-mail service to pass on information about a Chinese government reporting ban. He was identified when Yahoo officials disclosed information about his account.

Representatives of Google and Yahoo reached in London, and of Human Rights Watch in Paris, were unable to comment on the initiative on Tuesday morning.