A method for locating a quality of service problem implemented on a terminal during a multi-media session between two terminals over a communication network, which includes at least one media gateway that is able to retransmit media data packets without particular processing according to a “router mode” of management. The terminals obtain a first quality of service data item relating to the multi-media session. Following a change in the mode of management to a “relay mode” in which the multi-media session initially established between the terminals is split into two separate multi-media sessions, it includes: obtaining of a second quality of service data item relating to one of the two multi-media sessions established following the change in the mode of management of the media gateway; comparing the first and second quality of service data items; and locating the quality of service problem depending on the result of the comparison.