Disclosed is a method of detecting spoofing of a traffic alert and collision avoidance system, known as a TCAS, the TCAS having a Mode A, a Mode C and a Mode S for communicating with surrounding aircraft. The method includes: querying a suspected spoofing aircraft via Mode S of the TCAS and receiving a response to this query; deducing from the response at least some data, known as Mode S data, relating to the suspected spoofing aircraft; and validating Mode S data by querying the suspected spoofing aircraft via Mode A or Mode C of the TCAS.