An adaptation of Behrens' Method to the calculation of diffusion lengths in heterogeneous media is given. In all cases, the diffusion length in a medium containing absorbing lumps can be related to the self-shielding factor of the lumps. Calculations are presented only for the simplest case given but the results display considerable disagreement with a frequently used formula. On grounds which are mainly intuitive, it is believed that this method is more accurate, particularly for large moderator to absorber ratio. Final conclusions cannot be drawn, however, until more experimental data become available.