Boundary-layer analysis is performed for free convection flow over a hot horizontal surface embedded in a porous medium saturated with a gas of variable properties. The variable gas properties are accounted for via the assumption that thermal conductivity and dynamic viscosity are proportional to temperature. A similarity solution is shown to exist for the case of constant surface temperature. Numerical results for the stream function, horizontal velocity, and temperature profiles within the boundary layer as well as for the mass of entrained gas, surface slip velocity, and heat transfer rate at different values of the wall-temperature parameter are presented. Asymptotic solutions for large heating are also available to support the numerical work. Key words. Convection over a hot horizontal surface, boundary layers in porous media saturated with a gas of variable properties. boundary-layer velocity components in X and Y directions, Cartesian coordinates, boundary-layer coordinates, Howarth-Dorodnitsyn coordinate Symbols thermal diffusivity, heating or cooling parameter, small parameter, similarity variables, temperature, dynamic viscosity, density, stream function.