The adsorptive capacity of powdered activated carbon (PAC) is affected by the physicochemical characteristics of both the surface and sorbate as well as the pore-size distribution relative to sorbate size. The adsorptive capacities of PACs are commonly characterized using aqueous-phase indices such as iodine (I2), methylene blue (MB), p-nitrophenol (PNP), molasses, and tannin. PACs may also be characterized using gas adsorption with nitrogen or carbon dioxide to develop detailed pore-size distribution information. The primary purpose of this research was to examine 12 different PACs to determine the relationship between common aqueous-phase indices and true pore-size distributions using gas-phase indices. The results showed that the internal surface areas of PACs tended to be ordered according to the source materials as: wood > bituminous coal > lignite coal. It was also found that the coal-based carbons had a majority of their internal pores in the microporous range (<2.0 nm), whereas wood-based carbons tended to have a much wider pore-size distribution, well into the mesoporous range. The molasses and tannin numbers correlated well with a total pore volume of greater than about 1.1 nm. The MB and I2 numbers correlated well with the total surface area when micropores below 1.1 nm were also included. Surface functional groups were determined, using the Boehm titration methods, and were found to be less important in the sorptive capacity of PACs than the specific surface areas and pore-size distributions. It was proposed that the molasses-to-iodine ratio (MIR) and the inverse-tannin-to-iodine ratio (ITIR) may be useful as a means of estimating the pore-size distributions of PACs using only aqueous indices.