Capillary impregnation of a viscous liquid into carbon fiber bundles is investigated experimentally. Axial impregnation is examined in which flow occurs primarily parallel to the fibers' axes. For silicone oils, as well as for a curing epoxy system, the kinetics of axial impregnation follow closely h-alpha-t1/2, where h is the average displacement of the advancing front and t is the time of impregnation. The impregnation rates are higher than those predicted by theoretical models based upon simple geometries. The rates are strongly influenced by the pore size distribution in the fiber bundles. At a given overall average porosity, higher rates of impregnation are observed for samples having a higher pore heterogeneity than those with a narrow distribution of pore sizes. An analysis of the experimental data is presented which allows the effective pore size distribution to be calculated.