Employees' text messages are private, even when transmitted on devices their companies pay for, a U.S. appeals court ruled this week.

The 9th U.S. Circuit Court of Appeals ruled Wednesday in favor of a police officer and others who claimed that the city of Ontario, Calif., violated their Fourth Amendment rights, which provide citizens with a reasonable expectation of privacy, by reading the contents of their text messages. It also ruled that the company providing the department's paging service falls under the Stored Communications Act. That means carriers cannot reveal the contents of SMS or text messages without authorization from the end user -- even if the user is an employee.

In 2002, Ontario's police department issued pagers to its officers under a service plan that capped the number of characters each officer could send and receive via SMS. The department had a written policy banning personal use of its networks, computers, and communications devices, and claimed that policy encompassed pagers.

However, officers who went over the limit regularly paid the department back for overage costs. According to court records, the lieutenant in charge of pagers told at least one officer that he would not audit messages if officers paid the overage fees. The police chief said he decided to investigate whether the overage fees stemmed from personal use or work-related activities.

The department contacted the pager service provider, Arch Wireless, and requested transcripts of archived messages sent to and from Officer Jeff Quon, who had previously paid for overage costs and avoided audits. The investigation uncovered personal communication (with Quon's wife, among others). The personal text messages included statements that were sexual in nature.

Quon, his wife, a police dispatcher, and another member of the department sued, saying the city of Ontario in San Bernardino County engaged in unlawful search and seizure, while Arch Wireless violated the Stored Communications Act.

The ruling (PDF) stated that the police chief could have determined how much of the messages were personal by examining numbers or redacted records. The Fourth Amendment ruling was specific to the case and hinged on Quon's reasonable expectation of privacy, his attorney Dieter Dammeier said in an interview Thursday. In other words, the department's informal policy of accepting reimbursement and not auditing the communications led officers to expect privacy, which makes the search and seizure illegal.

The court also ruled that Arch Wireless was subject to the Stored Communications Act, which prohibits providers from releasing the contents of communications without consent from the sender or recipient.

Dammeier and other lawyers said that means cell phone carriers will fall under the act and therefore cannot legally release contents of texts and e-mails to companies without authorization from individual end users.

Employers cannot legally force employees to give up their rights under the U.S. Constitution, so policy acknowledgement forms giving consent would not protect employers, Dammeier said.

"From an employer's standpoint, the safest thing is just don't look at them," he said. "If they think there are overages going on because of personal use, there are other ways to find out without looking at the actual information. If there's a crime involved, they can always go to police and get a search warrant, but if it's just people being nosey, they don't need to see it."

This article was edited at 3 p.m. to include the attorney's statements.