The figure of merit for multilayer thermionic refrigeration is discussed in terms of an effective B factor, which has a similar definition as the B factor in thermoelectrics. We show that high efficiencies for cooling or power generation are only obtained with very high values of this B factor. Such high values can only be attained because of the low thermal conductivity of multilayers. The B factor for thermionics is usually less than the one for thermoelectrics. (C) 1999 American Institute of Physics. [S0021-8979(99)05824-7].