Convection in a cavity with a free surface and heated from the side is studied by a combination of flow visualization and particle image velocimetry. In these experiments, buoyancy and thermocapillarity are of comparable importance in driving the convection. The Prandtl number of the working fluid, the cavity aspect ratio, and the ratio of Rayleigh to Marangoni numbers are all held fixed; the primary experimentally varied parameter is the imposed temperature difference, which varied from 0.3 to 20 °C, resulting in a range of Marangoni numbers between 6×103and 4.2×105. For low Marangoni numbers, the flow is steady and two dimensional, as expected. The global nature of the flow is in good agreement with available numerical simulations of combined thermocapillary‐buoyancy driven convection. At higher Marangoni number, Ma>1.5×105, we observe a transition to steady three‐dimensional convection. The nature of this transition is typical of an imperfect bifurcation, and the flow structure is investigated both qualitatively and quantitatively. It is concluded that for the parameter values studied the first unstable mode consists of steady three‐dimensional, approximately cubical, vortical structures that are periodic along the axis of the cavity. The three‐dimensional flows observed by visualizations are in remarkable agreement with the recent numerical computations of Mundrane and Zebib [Phys. Fluids A5, 810 (1993)]. ©1996 American Institute of Physics.