Time dependent and independent measurements on a subcritical multiplying medium lead to the determination of the two group diffusion constants: k, L12, L22, l01, and l02. The measured values are related to these constants through five independent expressions. Similar measurements have been made on non uniform cylindrical lattices, in which the pitch is a function of the radius. In such lattices, the radial fluxes are expressed by Bessel function series, used to define a generalized expression for the effective multiplication factor.