For a heterogeneous multiplying system, a one-group diffusion model is shown to exhibit two spatial eigenvalues (inverse relaxation lengths). The smallest root, which describes the long-range behavior of the neutron flux, is that eigenvalue that would be exhibited by an equivalent homogeneous system. The largest root corresponds to the inverse relaxation length of the moderator. The existence of these two eigenvalues is relevant to interpretation of neutron noise measurements in boiling water reactors.