For the case of arbitrarily anisotropic scattering in monoenergetic neutron transport theory with no multiplication, the smallest root κ0 of the determinantal equation (which is equal to the inverse of the diffusion length) is considered as given by an infinite series in powers of the absorption parameter, where Σa is the macroscopic absorption cross section and Σ is the total macroscopic cross section. It is shown that αm depends only on the first m Legendre moments of the scattering probability.