The solubility and diffusion of water-acetic acid solutions in epoxy resins has been studied by nuclear magnetic resonance imaging (MRI) and spectroscopy (NMR), Fourier transform infrared (FTIR) spectroscopy, and light microscopy (LM). These techniques revealed the progression of a sharp diffusion front at a rate proportional to the square root of time. Both the swelling and rate of diffusion are dependent on acid dilution. At higher acid concentrations, hydrogen bonds were present, which has been interpreted as formation of acid dimers. The results indicate that molecular interactions play a major role in controlling the solubility and diffusion processes. The observation that voids can be filled up with penetrant strongly support the interpretation that molecular interactions rather than concentration gradient are responsible for these effects.