We have performed open‐tube diffusion of Zn inton‐type InP over the temperature range 525–675 °C. For heavily doped InP, the observed diffusion profiles are consistent with an interstitial‐substitutional model where the interstitial Zn is doubly ionized. However, for undoped InP the profiles have a significantly different shape and the diffusion is deeper. Assumption of neutral interstitial Zn is able to account for these two changes. A tentative model is proposed to explain the dependence of the interstitial Zn ionization state on the starting substrate dopant concentration.