The problem of buoyant-thermocapillary convection in cavities is governed by a relatively large number of nondimensional parameters, and there is consequently a large number of different types of flow that can be found in this system. Previous results give disjoint glimpses of a wide variety of qualitatively and quantitatively different results in widely different parts of parameter space. In this study, we report experiments on the primary and secondary instabilities for acetone as the working fluid with a Prandtl number of 4.44, and in a geometry with equal aspect ratios in the range from 1 to 8 in both the direction along and perpendicular to the applied temperature gradient. We thus complement previous work that mostly involved either fluid layers of large extent in both directions, or consisted of investigations of strictly two-dimensional disturbances. We investigate the qualitative and quantitative features of the fluid velocity field by flow visualization and particle tracking techniques. We observe the primary transition from an essentially two-dimensional flow to steady three-dimensional longitudinal rolls. The critical Marangoni number for this first transition is found to depend on the aspect ratios of the system, and varies from4.6×105at aspect ratio 2.0 to5.5×104at aspect ratio 3.5. The structure of the steady three-dimensional flow far above the transition is found to involve a nonintuitive reverse flow against the temperature driving due to the strongly nonlinear three-dimensional flow associated with the longitudinal rolls. Further, we have investigated the stability of this three-dimensional flow at larger Marangoni numbers, and find a novel oscillatory flow at critical Marangoni numbers of the order of6×105.We suggest possible mechanisms that give rise to the oscillations. ©1997 American Institute of Physics.