We have made the first phonon absorption measurements in the fractiona
l quantum Hall regime. Experiments have been conducted on both high mo
bility n and p-type GaAs/AlGaAs heterojunctions. In the n-type samples
used the electron densities were similar whilst the mobilities differ
ed greatly. The energy gaps determined by phonon absorption measuremen
ts give almost identical values, demonstrating that this method of mea
surement is rather insensitive to disorder. The values obtained at v =
2/3 are in good agreement with theoretical estimates of the magnetoro
ton energy gap. The results on p-type samples again demonstrate the re
latively small effect of disorder on the values obtained. The values a
re much lower than simple theoretical estimates. It is believed this i
s due to the larger influence of Landau level mixing in these samples.