Mass fluxes of ZnSe by physical vapor transport (PVT) were measured in the temperature range of 1050-1140°C using an in situ dynamic technique. The starting materials were heat treated by a hydrogen reduction process followed by the dynamic bake-out method. The amount and composition of the residual gas inside the experimental ampoules were measured after the transport experiments using a total-pressure gauge. The total residual gas pressures measured were about one order of magnitude lower than the values reported on similar processed ZnSe ampoules which were not treated with a hydrogen reduction process. As a result of the reduction in the residual gas pressure, the measured fluxes were 3-7 times higher than previously reported and correspond to a growth rate higher than 10 mm/day at 1120°C. The simultaneous measurements of partial pressures of transport species and the mass fluxes were also performed on a ZnSe optical ampoule. The partial pressures of Zn and Se 2 were obtained by measuring the optical densities of the vapor at the wavelengths of 2138, 3405, 3508, 3613, and 3792 . For the first time, the experimentally obtained mass fluxes were compared with those calculated, without any adjustable parameters, from a one-dimensional diffusion model which uses all the measured parameters (thermal field, partial pressures of transport species, and residual gas) as inputs, and good agreement was observed.