As-grown Fe-doped semiconducting InP wafers (residual carrier concentration ⩽1015 cm−3,estimated iron concentration5–8×1015 cm−3) were converted to semi-insulating, with high resistivity and good mobility, when annealed under appropriate conditions. This fact is very interesting, since it permits the preparation of semi-insulating InP with an Fe content substantially lower than that of standard as-grown material. In this article, we report the annealing parameters, together with the results of an extensive characterization (Hall effect,C–V,infrared absorption, and photoinduced current transient spectroscopy) of the treated samples. The onset of the semi-insulating regime seems to be primarily due to an annealing-related loss of shallow donors. ©1997 American Institute of Physics.