Thermoelectric power and conductivity measurements have been made as a function of temperature on a new nanostructured material, hydrogenated polymorphous silicon (pm-Si:H). The thermoelectric power is negative, so electrons are the dominant carriers. The activation energy of the thermopower is less than that of the dark conductivity. However, the short-circuit Seebeck current activation energy agrees with the conductivity-activation energy. These results are consistent with a model involving long-ranged fluctuations at the mobility edges. The magnitude of the fluctuations is larger than that measured in highly-doped hydrogenated amorphous silicon (a-Si:H) in relation to the peculiar structure of pm-Si:H. (C) 2000 Elsevier Science S.A. All rights reserved.