Yahoo China and MSN China confirmed on Friday that they had signed a code of conduct for their blogging operations that committed them to protecting the interests of the Chinese state.
The firms and other blog providers in China this week signed the "self-discipline" pact, under which they pledged to "safeguard state and public interests," a statement from the China Internet Society said.
The pact "encourages" Internet firms to register the real names, addresses and other personal details of the bloggers, and then keep this information.
The firms also committed to delete any "illegal or bad messages," according to a copy of the pact posted on the society's Web site.
Along with sex and violence, China's communist rulers have also deemed that opinions critical of it, as well as the spreading of democratic ideology, are not allowed.
Yahoo China and MSN China said they had signed the pact, but did not give further details.
"I can confirm that we signed the pact this week," spokesman Dou Xiaohan of Yahoo China said.
MSN China spokesman Feng Jinhu said: "We've signed the pact but there is no press release on that."
Some companies have caused uproar abroad for bowing to the Chinese government's demands by agreeing to censor Web sites and content banned by the nation's propaganda chiefs.
They have repeatedly insisted that they have no choice but to follow local regulations.
Yahoo has been slammed for passing on information to Chinese authorities about several users, including journalist Shi Tao (師濤), who got a 10-year jail sentence for divulging "state secrets."
Shi had sent a Chinese government order against media organizations discussing the 1989 Tiananmen Massacre, to an overseas Web site.
Press freedom watchdog Reporters Without Borders condemned the new blogging pact.
"The Chinese government has yet again forced Internet sector companies to cooperate on sensitive issues. In this case blogger registration and blog content," it said in a statement.