The effects of surface tension and buoyancy on the convection stability of fluid layers with a mean parabolic temperature distribution is examined. A variety of free-surface, thermal boundary conditions are used. Quantitative values for the effect on the critical Rayleigh number of the free-surface boundary condition, fluid depth, and physical properties are given. The role of the mean temperature profile is also discussed. The results should be of value in judging experiments where a free surface is used to enhance the flow visualization. The numerical procedure used to find the eigenvalues was effective and easily altered to obtain results for similar stability problems.