The onset of convection in a box of water‐saturated porous media is considered. The effects of viscosity variation due to large temperature differences are included. Critical conditions for three box geometries which are representative of the problem are reported: a thin vertical slab, a flat box, and a tall slender column. The critical Rayleigh number is found to be substantially reduced from the value predicted by constant viscosity calculation. The horizontal planform of the preferred cellular motion is found to be independent of the temperature difference, being a function of the aspect ratios of the box only. The liquid motion is faster near the lower hot surface since the viscosity decreases with depth. The case of very tall slender columns led to the study of a fourth‐order ordinary differential equation with a turning point at the boundary.