The observation of the local vibrational stretching mode of the manganese-hydrogen complex in GaP by infrared spectroscopy is reported. It is shown that when manganese is in the impurity regime, the complex only forms in very small concentrations in samples in which the Fermi level is about 0.3 eV above the valence band maximum. The reasons for this behavior are analyzed. It is argued that in GaMnP alloys, the formation of manganese-hydrogen complexes is more likely than in the impurity regime, but a full passivation of the manganese by hydrogen is not possible. However, the fraction of passivated manganese can impede ferromagnetism. (C) 2008 American Institute of Physics.