Chinese internet surfers have almost unfettered access to pornography, but news, health and education sites are routinely blocked, US researchers have found.
The team from the Harvard Law School tested more than 200,000 websites to find out what content was banned in China.
They found that nearly 20,000 sites were inaccessible, showing that China had an effective system of blocking sensitive material.
The findings provide an insight into the priorities for Beijing, which is promoting the use of the internet for business but trying to control it as a channel for political debate.
Professors Jonathan Zittrain and Benjamin Edelman, of the Berkman Center for Internet and Society at Harvard, spent eight months investigating how China controls the net.
They found that Beijing actively polices content on the web, intermittently blocking some general-interest high-profile sites whose content changes frequently, such as the tech website Slashdot.
There also seemed to be an ambivalent attitude to porn on the net. The researchers found that China blocked just 13.4% of their sample of well-known sexually explicit sites.
"Blocking of such sites as Playboy and Penthouse suggests a purposeful decision to restrict sexually explicit material," said the researchers.
"Yet the well-known sites of Hustler Magazine and whitehouse.com were consistently accessible."
Most of the material blocked were sites offering information about news, health, education, and entertainment, as well as thousands of sites from Taiwan.
The researchers found that BBC News was consistently unreachable, while CNN, Time Magazine, the Miami Herald and the Philadelphia Inquirer were also often unavailable.
Other sites with potentially sensitive political material were also blocked, including dozens of sites about democracy and human rights.
But the concerns of Beijing extend beyond just politics. Chinese surfers eager to find out more about Aids or other sexually-transmitted diseases would be left wanting.
The Harvard team found sites such as the Aids Healthcare Foundation, the Internet Mental Health resource and the Health in China research project were inaccessible.
Even seemingly harmless sites run by well-known institutions of higher education, including Caltech, Columbia University and the Massachusetts Institute of Technology, were blocked.
"The Chinese Government and associated network authorities are clearly continuing to experiment with different forms of blocking," said the researchers.
This indicated that "network filtering is an important instrument of state internet policy, and one to which significant technical and human resources continue to be devoted."
For their report, Professors Zittrain and Edelman used two different methods. The first involved making an international phone call and dialling up Chinese internet service providers and going online.
After a few weeks they were unable to make a connection. So they then connected to open proxy servers in China to test access to sites.
What they discovered was at least four distinct and independently operable methods of internet filtering.
The most common means of blocking was at the router level, based on the address of a site, meaning that a whole site was often blocked, even if only a small part of it was considered unsuitable.
But China appears to be becoming more sophisticated in its controls on the net.
"Nascent but growing forms of filtering appear to be targeted to limit the information that can be gleaned from search engines, enabling the automated blocking of search results that may not, yet, have been filtered through human placement on a 'forbidden' list," said the researchers.