Bogdan Barbulescu was fired in 2007 for using Yahoo Messenger not only for work but also to chat with his fiancee and brother.
The Grand Chamber of the Strasbourg-based court heard the case of Bogdan Barbulescu, a 37-year-old Romanian sales engineer.
His employer fired him in 2007 after discovering he was using Yahoo Messenger not only for work but also to chat with his fiancee and brother.
In January this year, the ECHR dismissed Barbulescu's argument that the company had violated his right to confidential correspondence but he succeeded in having the case referred back to the court.
Barbulescu maintains that his employer invaded his privacy by spying on his communications which, the court said, included messages "relating to personal matters such as his health and sex life".
A lawyer representing the Romanian government said the engineer had been well aware of the firm's rules prohibiting the use of company resources for personal purposes.
"He knew all about this ban because he had been informed about it," lawyer Catrinel Brumar said.
She said the company had no interest in the content of the messages, only in the fact that they were private.
But Emeric Domokos-Hancu, representing the applicant, argued he had been unaware his online exchanges were being monitored because the company had used "spyware" software.
In a written submission to the judges, the European Trade Union Confederation said the case was "of particular importance for workers' protection in the digital age".
The ruling was deferred and will be delivered in several months' time.
It will be definitive and will be eagerly awaited because the ECHR's decisions are binding on the 47 countries that have ratified the European Convention on Human Rights.