Deep donor to acceptor transitions between interstitial and substitutional Zn in InP are investigated in this study via the photoluminescence technique. Interstitial Zn is to be fully activated when treated by rapid thermal annealing process at 600-degrees-C. When treated at 400-degrees-C, however, interstitial Zn is partially activated so that medium distance deep donor-to-acceptor transitions become less available. This medium distance transition is estimated to be in a range between 35 and 60 angstrom.