Magnetic resonance imaging (MRI) is used to probe the evolution of geometric characteristics such as the volume, shape, surface area, and cluster size of octanol ganglia trapped in a model porous medium, in this case a packing of spheres, as they dissolve into a mobile aqueous phase. The resulting pore-scale information is used to assess various assumptions used in existing models of the dissolution process. Dissolution of the ganglia was characterized by a reduction in the overall number of ganglia with little effect on the shape and mean of the volume distribution of the ganglia. This apparently anomalous result is explained by dissolution of the ganglia until they reach a critical size, which is dependent on the structure of the pore space, at which point they are mobilized and subsequently removed from the porous medium. The shape of the entrapped ganglia is characterized by a fractal dimension in the range 2.2-2.3, suggesting that models which assume a Euclidean geometry for the entrapped ganglia are appropriate. No significant change in the shape of entrapped ganglia is observed during dissolution. In agreement with the results of earlier workers, most hydrocarbon ganglia exist as singlets within the pore structure. Copyright 2000 Academic Press.