Users of Microsoft's new China-based Internet portal were blocked from using the words "democracy", "freedom" and "human rights" in an apparent move by the US software giant to appease Beijing.
Other words that could not be used on Microsoft's free online blog service MSN Spaces include "Taiwan independence" and "demonstration".
Bloggers who enter such words or other politically charged or pornographic content are prompted with a message that reads: "This item should not contain forbidden speech such as profanity. Please enter a different word for this item".
Officials at Microsoft's Beijing offices refused to comment Monday.
Internet sites in China are strongly urged to abide by a code of conduct and self-censor any information that could be viewed by the government as politically sensitive, pornographic or illegal.
For many Chinese websites, such content also includes news stories that the government considers unfavorable or does not want published.
New regulations issued in March now require that all China-based websites be formally registered with the government by the end of June or be shut down by Internet police.
Microsoft formed a joint venture with China's state-funded Shanghai Alliance Investment Ltd (SAIL) last month to launch the MSN China web portal.
Microsoft is not the only international tech company to comply with China's stringent Internet rules.
Yahoo! and Google -- the two most popular Internet search engines -- have already been criticized for cooperating with the Chinese government to censor the Internet.
The Paris-based Reporters Without Borders (RSF) earlier said it "deplores the irresponsible policies of United States Internet firms Yahoo! and Google in bowing directly and indirectly to Chinese government demands for censorship".
An RSF spokesman said Monday the group was checking to see if Microsoft had followed suit.
"We are checking into this. If it is correct, it proves once again that US companies are actively collaborating with the Chinese government's censorship efforts," the spokesman told AFP.
"We strongly condemn that."