Experimental investigations were conducted in a cubic air filled cavity (L = 1.04 m, H = 0.94 m, P = 0.70 m). The cavity was equipped with two opposing differentially heated vertical walls maintained at uniform temperatures, with two insulated horizontal walls and two adiabatic lateral vertical walls. The experimental apparatus, designed to obtain well controlled boundary conditions, is reported in details. An examination of temperature fields in the vicinity of the ceiling and the floor allowed the characterization of the actual boundary conditions on these walls. An analytic law was determined, giving the temperature distribution along each horizontal wall. Flow visualizations, using laser tomography and spectral analysis of time dependent signal of the temperature recorded at several characteristic points in the cavity, were performed. It pointed out the existence of unsteadiness and revealed a complex interaction between internal gravity waves, thermal instabilities along the floor and Tollmien-Schlichting waves in the hot vertical boundary layer. Heal transfer along the active walls was evaluated from temperature measurements taken very close to the walls.