An experimental investigation is described of the effect of stabilizing thermal gradients on natural convection in silicone oils in rectangular enclosures with different aspect ratios. The Prandtl numbers are of the order of 105, Grashof numbers range up to 20, and the aspect ratios are 1 and 3. The thermal boundary conditions are established by imposing different temperatures on opposite walls of the enclosure so that there is simultaneous horizontal and vertical heat flow. The effect of stabilizing temperature gradients on flow established by horizontal gradients and the effect of horizontal temperature gradients on a stably stratified fluid are studied for ranges of the parameters. Streamline patterns are observed at steady-state and velocity profiles are calculated from streamline data and extrapolated with approximate theoretical calculations. It is found that the flow generated by a horizontal gradient is retarded by a stabilizing thermal gradient. The reduction is shown as a function of the relevent parameters. For the range of variables investigated complete stabilization of the fluid driven by a horizontal gradient does not seem possible by means of a vertical gradient. The steady state flow patterns obtained do not depend on the manner in which the flow is started, i.e., on the order in which the temperature differences are imposed.