Abstract In thermal DLTS spectra of GaP and GaAs0.1P0.9 we often found two emission levels with acceptor-like character (σp⪢σn) which appeared with comparable peak amplitudes. Studying the electron capture behaviour, the concentration profiles and the field influence on the thermal emission rates, we proved that both levels are related to the same impurity centre. We measured the photo cross sections in a wide spectral range by optical DLTS combined with electrical pulses. The results allowed us to identify the impurity centre as the isolated FeGa double acceptor, supporting and completing earlier results of Yang et al. [8] on the lower acceptor level in GaP. With our results we falsify an alternative model which assumes a complex ZnGaVP as responsible for the lower level.