Theoretical and experimental approaches have been developed for predicting drying rates of aqueous ink images printed on paper by ink jet printing technology. The predictive aspect of the model requires that the diffusion constant be known for each ink—paper system as a function of concentration and temperature, and that evaporation rates be measured. Experimental data input to the theoretical model is obtained via a dynamic sorption apparatus which enables the in situ study of individual microscopic ink jet drops as they spread and penetrate into a porous structure. Simultaneous plan- and side-view video recording coupled with an image analyser enables rapid data acquisition and processing of the drop profile and developing contact line. Smooth, non-porous, transparent polymer films and a range of papers with different absorptivity have been investigated. The salient features of the diffusion model and its applicability are discussed in light of the experimental results.