Experimental results are reported for free and mixed convection in liquid saturated, horizontal porous layers with localized heating from below. Three different sizes of heat source have been considered which give a relative heater size (axial length/layer depth) of 1.0–5.0. For natural convection, Nusselt numbers have been obtained for Rayleigh numbers of practical interest, 1 ⩽ Rah ⩽ 1000, while for mixed convection, results cover 1 ⩽ Rah ⩽ 1000 and 0.1 ⩽ Peh ⩽ 120. Based on dimensional analysis and nonlinear regression, correlations for Nusselt number against Rayleigh and Peclet numbers have been obtained. Experimental Nusselt numbers compare very well with numerically calculated values. Improved agreement between calculations and measurements can be obtained through use of an effective thermal conductivity. The stability of the flow and temperature fields are also carefully examined. The existence of a Peclet number for which the overall heat transfer rate is a minimum is confirmed by experiment.