The travelling wave technique was used to measure the drift mobility of photoexcited carriers in a-Si:H from 1·6 to 300 K. Above 100 K the drift mobility increases with temperature as expected from multiple trapping theories. Below 100K the measured signal increases with decreasing temperature reaching values that, if interpreted as drift mobility, exceed 07middot;2 cm2V−1s−1at 1·6 K. The results are qualitatively consistent with the large drift mobilities measured at low temperatures by the time-of-flight technique. The drift mobility increases linearly with travelling wave frequency below l00 K but is independent of frequency above 100 K. However, the interpretation of the low-temperature signal as drift mobility is in doubt because the sign of the signal below 100 K, which should indicate the sign of the dominant carrier, is instead determined by the orientation of theyaxis of the LiNbO3plate used to generate the travelling wave. This indicates a heretofore unknown interaction of the travelling wave with the semiconductor that is dominant below 100 K.