The T-matrix method has been used to calculate scattering cross-sections of two spherical particles close to each other. By comparing our results with those expected for infinite distance, we can determine the maximum distance required to produce interparticle lightscattering interactions. As a result, we can determine the degree of dilution needed in a particle suspension to remain within the single-scattering approximation. Cross sections have been calculated for a relative refractive index m = 1.2 and a range of particle sizes and separations. Since the approach in this paper assumes double scattering as the first mechanism for beginning multiple scattering, our results will be compared with those arising from the condition that the optical depth is less than unity, tau < 1. It is shown that this condition, widely used in transfer theory, is more restrictive than ours except for the smallest particles, where double scattering becomes the main agent for multiple scattering.