The stability of buoyancy driven shear hows in inclined long cavities with end wall temperature difference is investigated for different inclinations and a wide range of Prandtl number. The results of the linear stability analysis show that the basic unicellular motion may break down due to stationary or oscillatory instabilities. The stationary rolls are nearly square and the mechanism of this instability is mainly hydrodynamic. The oscillatory instability is driven by buoyancy and consists of long-wave rolls of about 10 times the width of the cavity. For Pr < 0.2 stationary modes are the most unstable while for Pr > 0.7 oscillatory modes are preferred. At moderate Prandtl numbers (0.2 < Pr < 0.7) the most unstable perturbation is determined by the angle of inclination. A better understanding of the instability mechanisms is provided by an energy analysis of the marginally stable perturbations. Results from direct numerical simulations of the full non-linear unsteady equations in closed configurations are also presented. Both stationary and oscillatory instabilities have been obtained and their characteristic features (wave number and frequency) are consistent with the linear theoretical predictions.