Phase equilibrium and mutual diffusion in the poly(sulfone) (PSF)-dimethylsulfoxide (DMSO) system have been studied at temperatures from 20 to 110-degrees-C over a wide solution composition range. The phase diagram for this system has been obtained and the effect of water on boundary concentrations has been studied. It is shown that the presence of water in DMSO has a considerable effect on the binodal curve. Thus, increasing the water content up to 1.3% by weight results in the displacement of the upper critical solution temperature by 30-degrees-C and the widening of the two-phase region. The kinetic regularities in mutual dissolution of components have been investigated. The concentration dependencies of the mutual diffusion coefficients in the systems studied are presented. The effect of moisture in DMSO on the mutual diffusion coefficients appears near the phase transition in the system.