Abstract A detailed experimental study of the heat transfer in an open isothermal cubic cavity for mixed natural and forced convection is reported. First the experimental model with a temperature control system, the experimental procedure, and the method for determination of the convective heat losses from the experimental data are presented. After presentation of the experimental conditions for the three models of different sizes that were tested, dimensionless parameters (Gr, Re, and Nu numbers) are introduced to generalize the results. In this way, the main influence of natural convection can be separated, and, using regressional analysis, a general interdependence among Nusselt, Reynolds, and Grashof numbers can be obtained. The local heat transfer effects are indicated by maps of local heat transfer coefficient distributions in the cavity for different orientations of the model. To reduce the effects of convective heat transfer, the influence of different types of front masks were analyzed. The usefulness of the experimental results obtained is shown by estimating the convective heat losses for the central solar receiver of the French solar power plant THEMIS.