It is shown that in the propagation of shock waves through a gas-mixture diffusion produces effects similar to those of viscosity and thermal conduction. If the molecular weights of the two gases are not very different, the effects are small: when they are as widely different as those of hydrogen and oxygen, the diffusion effects are more important than those of thermal conduction, and are comparable with those of viscosity. The velocities of diffusion involved are an appreciable fraction of the velocity of sound in the gas.