Abstract In a frequently applied method the exponent, n, of the distance R, of interacting molecules, appearing in the expression of the efficiency, f, of transfer, is determined from the slope of a straight line obtained by plotting In ( f −1 −1) vs. In c c 0 ( c and c 0 are the concentration of the solution and a constant critical concentration characteristic of the interacting molecules and their environment, as defined in the Förster theory of transfer). The dependence of f on c is usually determined experimentally from the concentration quenching of fluorescence. From the theoretical quenching curve and the analytical expression for the slope, it is concluded that In ( f −1 −1) vs. In c c 0 is not strictly linear, but a curve with slopes yielding exponents from n = −6− n = −3, in contrast to the fact that the interaction theoretically remains very weak (with n = −6). A correct exponent is obtained experimentally from the high-concentration part of the quenching curve or by using the theoretical dependence of the slope on the concentration in the case of very weak interactions. For strong interactions, In ( f −1 −1) vs. In c c 0 is linear, and n = −3 in the whole concentration range. However, f is slightly volume-dependent.