Terahertz pulsed imaging (TPI) was used to analyse the diffusion of water into hydroxypropyl methyl cellulose (HPMC) matrices as well as acrylate copolymer matrices in reflection. Using TPI it is possible to monitor the phase changes that occur in the polymer matrix, investigate the growth kinetics of the process as well as follow the water diffusion front as it propagates through the porous polymer matrix.