The acoustoelectric method of determining carrier mobility &mgr; in semiconductor films has been modified to permit its application to high‐resistance, low‐mobility films. The conventional method requires knowledge of the acoustic loss caused by the mobile carriers; in high‐resistance films this loss becomes too small to be measured. We show that the required information may be derived from knowledge of the acoustic power and the device geometry. Our samples were amorphous hydrogenated Si and Si0.6Ge0.4films on nonpiezoelectric substrates, separated by a convenient air gap (12.5 &mgr;m) from a LiNb03slab carrying surface acoustic waves. One sample had 108&OHgr;/&laplac; and &mgr;=0.08 cm2/V sec, another sample 1010&OHgr;/&laplac; and &mgr;=0.5 cm2/V sec.