The kinetics of compound formation in the molybdenum-selenium system has been investigated using elementally modulated reactants to control overall composition and diffusion length. We observed the facile formation of MoSe2 at low temperatures when the composition was above 50 atom % selenium. No evidence was found for the low-temperature formation of the other known stable molybdenum selenide, the cluster compound Mo6Se8. When the composition of the initially modulated reactant was close to 25% selenium, a previously unreported compound was observed to form. This new compound, Mo3Se, has the A-25 crystal structure. The superconducting transition temperature appears to be very sensitive to composition, with a sharp resistive transition at 7 K in one sample and a sharp diamagnetic transition observed in a second sample at 2.2 K. The kinetics of phase formation in this system is analyzed in terms of nucleation kinetics.