The buoyancy driven thermal stability of a radiating nongray gas between two infinitely long vertical plates is studied analytically. The Squire theorem is shown to hold, and two‐dimensional disturbances are more dangerous than the corresponding three‐dimensional ones. A combination of the Galerkin and Chandrasekhar methods are used to solve the eigenvalue problem associated with two‐dimensional disturbances. In contrast to the Benard problem, the effect of radiation on the onset of stationary cells diminishes as the optical thickness of the gas increases indefinitely.