A one-dimensional, slice-selective magnetic resonance imaging (MRI) technique is used to image in situ the penetration of water into poly(ethylene oxide) (PEG). Spin-lattice relaxation time (T-1)-weighted imaging experiments provide simultaneous, quantitative measurements of polymer and water concentration profiles, penetrant front motion kinetics and swelling kinetics. The propagation of the penetrant front is observed to be a diffusion-controlled process, with no volume change on mixing during swelling. Excellent agreement is observed when comparing the polymer concentration profiles obtained from the T-1 analysis with those measured by H-1 imaging of PEO during penetration by D2O, under which conditions only the polymer response is imaged.