Dissolution and mutual diffusion of poly(N-vinylpyrrolidone) (PVP) in short-chain poly(ethylene glycol) PEG400 were studied by wedge microinterferometry over the temperature range of 40-100degreesC. Successive photographs of interference patterns measured at lambda = 546 nm with an optical microscope at 130x magnification were used to determine the PVP/PEG concentration-distance profiles. These profiles were found to be highly asymmetric, exhibiting steep concentration gradients near the surface of the glassy polymer sample. The PVP/PEG system is completely miscible, and interdiffusion kinetics are Fickian with a concentration-dependent mutual diffusion coefficient, D-V. Thermal activation of diffusion was studied in terms of an Arrhenius-type relation, with concentration dependent activation energy E-n. Values of D-V and E-a are in accord with the compositional behavior of the glass transition temperature in PVP-PEG blends, indicating that PVP plasticized with PEG behaves like an elastomer.