Laminar boundary‐layer heat transfer from a partially ionized monatomic gas to a highly cooled wall with no applied external electric or magnetic fields is analyzed by the similarity solution approach. The gas is assumed to be electrically neutral with ambipolar diffusion by ion‐electron pairs. In addition, equal electron and atom‐ion temperatures are assumed, and sheath effects are not considered. From existing perfect gas similarity solutions, the number of parameters that need to be investigated is reduced to those directly related to ionization effects. Solutions for the limiting cases of a large diffusion rate compared to the net production rate of ions and for local equilibrium are obtained for a range of Prandtl and Lewis numbers. An approximate prediction method that includes variable property considerations is then suggested to account for variable free‐stream velocity and ionization effects in a flow over a highly cooled wall.