This study examines the influence of insoluble surfactant on the steady thermocapillary flow in a rectangular cavity with an upper, deformable free surface and differentially heated side walls. The numerical solutions are obtained with a finite difference method, together with a boundary-fitted curvilinear coordinate system. The results show that the thermocapillary convection can be stabilized by the addition of an insoluble surfactant. Furthermore, as the concentration of the surfactant increases (higher elasticity number), oscillatory instability caused by that surfactant may occur when the slope of the surface tension becomes negative. For higher Peclet numbers, the concentration boundary is formed near the cold wall, and the clean surface appears in the region near the hot wall. Oscillatory flow may also be induced by a high local surfactant concentration that is created by a concentration boundary.