In studying the radiationless energy transfer between unlike molecules (heterotransfer) in fluid and rigid solutions the fluctuations of the concentration of the acceptor molecules, as well as the dependence of the probability of resonance excitation energy transfer on the mutual orientation of the transition moments of the interacting donor and acceptor molecules have been taken into account. With these and the assumptions of the shell model of a luminescent centre (A. Kawski and J. Kaminski, Z. Naturforsch. 29 a, 452 [1974]) one obtains the Förster expression for the quantum yield of the donor fluorescence quenched by foreign absorbing substances
