The senators also warned that if the companies don't act voluntarily they could face legislation that would prohibit them from cooperating with repressive regimes.
"It is critical to the protection of fundamental human rights that a voluntary code of conduct be finalized and implemented as soon as possible," the senators wrote in the letter, which was sent Monday to Microsoft CEO Steve Ballmer, Yahoo CEO Jerry Yang, and Google chief Eric Schmidt.
The senators noted that discussions with the companies on adopting such a code that were held in May have thus far yielded nothing.
"At the hearing we agreed that Google and other American Internet companies operating in Internet-restricting countries should promote free speech and not facilitate repression," the letter said.
"We recognize that the code of conduct raises complicated issues with potentially far-reaching effects on your company's operations, but with everyday that the code is not finalized the human rights of people across the globe are jeopardized," the senators continued.
Durbin and Coburn have led a congressional effort to encourage U.S. tech companies not to cooperate with foreign governments that seek personal information on dissident bloggers and other Internet users. Lawmakers have stopped short of passing legislation that would prohibit such cooperation.
That could change. "If American Internet companies are unable to regulate themselves effectively, Congress may be forced to consider doing so," the senators warned.
Yahoo has been widely condemned for handing over information on dissident Shi Tao to the Chinese government. He was ultimately convicted and sentenced to 10 years in prison for forwarding an e-mail to a human rights group. Internet companies have countered that they are required to obey the laws of countries in which they operate.
The senators also said they were concerned that, without a code of conduct in place, Internet companies will be pressured to disclose to Chinese authorities personal data on athletes, journalists, and tourists visiting China for this summer's Olympic Games.