This article deals with an experimental study of air free convection in a large cavity (0.94 m height, 1.04 m width, 0.70 m depth), equipped with two opposing differentially heated vertical walls : the other being insulated. The temperature difference between the two active walls is 20 degrees C, which yields to a Rayleigh number value, based on the height, of 1.69 x 10(9). A detailed analysis of the local temperature and velocity fields is performed. The vertical distribution of temperature in the central part of the cavity emphasises the centro-symmetrical property of this region and allows to distinguish five linearly stratified zones. In the same way, the study of the ascending and descending thermal boundary layers points out a great symmetry of the profiles, with a phenomenon of relaminarisation, followed by the existence of a weakly turbulent thermal field. In contrast, the examination of the velocity field clearly reveals a difference in the behaviour of the ascending and descending dynamic boundary layers with an earlier development of the turbulence along the hot wall than along the cold one.