This paper deals with thermal instabilities occurring in a mixed convection phenomenon for a flow in a uniformly heated horizontal duct. These instabilities manifest themselves through the appearance of large amplitude temporal fluctuations in the wall temperature. The physical mechanism governing these instabilities has been described elsewhere, so here a thermodynamical concept is proposed which allows justification of the occurrence of such fluctuations. This concept is based upon the representative temperatures of the system and the reference temperatures, It is shown how a system which is represented by only one temperature, and containing two reference temperatures. adopts a non-stationary behaviour in order to eliminate sporadically one of the two reference temperatures.