For the purpose of studying a heterogeneous multiplying medium, the author proposes dividing it into a number of homogeneous regions having the diffusion and absorption properties of the pure moderator. The fuel elements, represented by portions of active surfaces of zero thickness, constitute the separation surfaces of these sub-regions. Externally, the system is bounded by passive surfaces devoid of fissionable nuclei. The theory of diffusion involving several groups of neutrons is applied to each sub-region, while the productive and absorbing effects of fissionable materials are represented by the conditions on the active surfaces. To apply the method, it is necessary to know certain parameters of the behaviour of an active surface in a known flux. The moderator group constants are presumed to be known. The author shows that, theoretically, a single exponential experiment, carried out with a very small number of rods, should suffice to determine these parameters experimentally. The facility used for these experiments is a subcritical assembly; the fuel is uranium oxide containing 1.8% uranium235 ; a water moderator is used. Measurements made for a series of different configurations confirm that the parameters sought depend solely on the nature of the fuel. The results are used to forecast the behaviour of a subcritical and a critical lattice. In the first case the calculations are verified directly by experiment; in the second, they are checked by comparison with the published results.