The subgap optical absorption of GaAs layers grown by low temperature molecular beam epitaxy is measured by photothermal deflection spectroscopy (PDS). The absorption increases as the growth temperature decreases at a fixed wavelength. Defect densities evaluated from the absorption spectra and the known absorption cross sections are between 1018and 1019cm−3. It is shown that complementary PDS phase spectra can be used to separate the absorption of the epitaxial layers from the bulk. ©1995 American Institute of Physics.