Attenuated total reflection infrared spectroscopy was used to measure interdiffusion at a model glassy/rubbery interface consisting of polystyrene (PS) below its glass transition temperature in contact with poly(vinyl methyl ether) (PVME). PVME swelled PS at temperatures ranging from 60 to 95-degrees-C, corresponding to 41 to 6-degrees-C below the glass transition of PS. This swelling was confirmed with dynamic mechanical analysis using glassy crosslinked PS in contact with PVME. The swelling process was characterized by an interfacial velocity as the PVME swelled PS. The relaxation time for the swelling process was determined from the interface velocity as a function of temperature and the results indicated that the swelling process is controlled by the relaxation time of the slowly diffusing component (PS). The relaxation time ranged from 141 min at 60-degrees-C to 16.8 s at 95-degrees-C. The activation energy for the relaxation of the PS matrix was determined as 40.9 +/- 2.6 kcal/mol using the Arrhenius expression, in good agreement with values reported in the literature for the beta relaxation of PS which are in the range of 35-40 kcal/mol.