Solvent diffusion into polymers is central to their performance, for example as controlled delivery pharmaceutical products. In this work, we present the novel application of terahertz (THz) time-domain reflectance spectroscopy to quantify the ingress of acetone into various polymer materials. Reflections from the top and bottom surfaces of the discs are temporally resolved, as is a reflection from the interface between acetone-wet polymer and dry polymer. These reflections enable both the polymer swelling and more importantly the position of the liquid diffusion front to be quantified as a function of time. Verification of these measurements is provided by equivalent magnetic resonance imaging (MRI) measurements, with which there is excellent agreement. (C) 2007 Elsevier Ltd. All rights reserved.