Google Talk, the US tech giant's messaging and telephony service, was knocked offline on Thursday for unknown reasons, but was restored after more than four hours, the company said.
A Google status page said the service appeared to be malfunctioning around 10.40am GMT, with the outage continuing more than three hours later.
"We're aware of a problem with Google Talk affecting a majority of users," the status page said.
"The affected users are able to access Google Talk, but are seeing error messages and/or other unexpected behaviour."
At 1.40pm GMT, the page said, "We are continuing to investigate this issue."
At 3.23pm GMT, Google said, "The problem with Google Talk should be resolved," but offered no additional details.