The diffusion of methyl ethyl ketone (MEK) in two vinyl alcohol/vinyl butyral (VA/VBu) copolymers (11- and 19-wt. % VA) was studied at low vapor activities using Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. MEK has the ability to interact through hydrogen bonding to sites in the polymer- and was chosen to study the effect of penetrant-polymer solvation on molecular diffusion. The assumption of local equilibrium was verified by examining the time-evolved concentrations of hydogen-bound and free MEK determined from the carbonyl (C = O) stretching bands. A mathematical model that accounted explicitly for solvation during the diffusion process was developed Solvation hinder ed the diffusion of MEK in the VA/VBu copolymer by factors of 2.0 (11-wt. % VA) and 2.6 (19-wt % VA). After separating solvation front diffusion true diffusion coefficients of MEK were compared to those of methylene chloride (CH2Cl2), a noninteracting penetrant of similar size to MEK. Within experimental error, true diffusion coefficients for MEK were the same as those for CH2Cl2 over the concentration range studied.