Experiments showing the effects of uniaxial stress on the structure of the zero-phonon line (ZPL) obtained in the optical absorption spectrum from a ion in GaP are described and interpreted using a new theoretical model. It is found that the components of the ZPL are split when the stress is applied along the , and axes. These results together with the spectra observed without applied stress, show that previous energy level diagrams for this centre cannot be correct. A vibronic model is used to describe this system which includes coupling to e modes in the ground states, to both e and modes in the excited excited states and to modes between and . The model is shown also to give a good explanation of the published Zeeman data obtained from photoluminescence measurements as well as the new uniaxial stress data presented here. A brief discussion is given of the type of Jahn-Teller effect operational in this system emphasizing that it is not necessarily of the orthorhombic type in the excited state.