High resolution photoluminescence spectroscopy at 4.2 K and at a magnetic field of 9.7 T is employed to resolve and identify residual shallow donors in liquid phase epitaxial InP. Sulphur is found to be the dominant inadvertent contaminant in high purity material prepared from growth solutions which have undergone long baking treatments. Silicon is also observed in most samples, but at significantly lower levels than sulphur.