We report on novel technique for determining transport parameters in a-Si:H using a waveguide structure. The technique is based on measuring the screening of an applied voltage by photocarriers within the guide. The results, which are analyzed by rate equations which balance recombination, drift and diffusion currents, yield electron and hole mobilities of 0.6 and 0.2 cm2/(V sec) respectively. The hole mobility is close to its microscopic value. © 1991 Elsevier Science Publishers B.V. All rights reserved.