A microvisual flow cell was used to observe the effects of fluid viscosity ratio and of pore size distribution on the mechanisms of steady cocurrent two-phase flow in porous media. The transition in flow mechanisms was found to have the same dependence on capillary number, N\sbca, for all viscosity ratios provided N\sbca was defined in terms of the interstitial velocity of the wetting phase. A simple theoretical model of ganglia flow through an idealized pore constriction was developed. Although velocity varied greatly during ganglion passage, the average volumetric flow rate of the nonwetting phase agreed well with relative permeability theory. A series of displacement experiments were performed with several oil-water-alcohol systems where diffusion occurred between oil and water phases. The amount of spontaneous emulsification observed was found to be greatest in systems where both Marangoni flow and local super saturation due to diffusion were expected.