The optical absorption associated with charge carriers in NiO and CoO has been obtained from transmission measurements on pure and Li-doped crystals in the wavelength region 05 to 700 μm, at temperatures between 100 and 500°K. The absorption increases smoothly with frequency to a maximum near 1 ev and then shows a further increase extending to 2 ev and higher. In the region from 05 to about 4 μm the absorption increases slowly with temperature and correlates with the Li concentration. In the far infra-red the absorption σFI is independent of frequency and, for T σdc. The activation energy associated with σFI is smaller than the d.c. value. The behaviour between 1 and 700 μm is very similar to that predicted by the theories of Klinger, Reik et al. and Bogomolov et al. for non-diagonal (hopping) transitions by small polarons. Similar absorption in TiO2 and other 3d oxides between 1 and 10 μm has been attributed to a hopping component in the free-carrier mobility. In NiO and CoO, however, we conclude that the absorption is primarily due to small-radius polarons hopping around Li centres with high-temperature activation energies Ea of 02-025 ev and 03-04 ev respectively. Above 400°K in NiO, σFI -> σdc and electrical studies indicate that about 20-30% of the carriers are ionized. No new features appear in the absorption but the above Ea values give an upper limit for the hopping energy of any non-diagonal component in the d.c. mobility. We infer a relaxation time of less than 10-13s for the diagonal component implying a bandwidth of at least 5 mev. Recent d.c. mobility data support a band picture, but a closer analysis indicates that an activated mobility with Ea less, similar 02 ev cannot be excluded.