Time-resolved near-resonance photon scattering in a gas is expressed in terms of a triple convolution of the photon counting rate from a single resonance (characterized by a total width which contains additive contributions from radiative decay, spontaneous predissociation, and collisionally induced predissociation) with a Lorentzian profile, which incorporates the effects of collisionally induced intrastate cross relaxation and phase shifts, and with a Doppler-Gaussian profile. This theory provides a quantitative picture for the recent experimental data of Rousseau, Patterson, and Williams [Phys. Rev. Lett. 34, 1306 (1975)] on photon scattering from collisionally perturbed molecular iodine.