Magnetic resonance imaging (MRI) is used to visualize the dissolution of entrapped ganglia or "blobs" of octanol within the pore space of a randomly packed bed of glass ballotini, by a mobile aqueous phase. MRI provides three dimensional images, able to distinguish the solid, hydrocarbon, and aqueous phases, as well as velocity maps of the mobile aqueous phase. Dissolution of the hydrocarbon phase has been modeled using a one dimensional advection-dispersion description incorporating a mass transfer term between the hydrocarbon and aqueous phases. Essential to this mass transfer term is a description of the interfacial area between the hydrocarbon and aqueous phases which is actively involved in dissolution and which can be determined directly from the images. The experimental data are best modeled by evaluating an effective interfacial area term characterizing the hydrocarbon/water boundary which excludes the narrowest constrictions within the interparticle space. MRI visualizations of the structure of the pore space and the flow processes occurring within it, demonstrate that heterogeneities in the flow at the length-scale of individual pores within the interparticle space cause significant heterogeneity in the dissolution process which becomes significant at low hydrocarbon saturations.