In this work we report on the effect of annealing high-resistivity InP grown by gas source molecular beam epitaxy in the presence of an electron cyclotron resonance generated He plasma. Previous work has shown that InP grown using this technique exhibits resistivity greater than 105 Ω cm and Be-doped InGaAsP (lattice matched to InP, band-gap wavelength 1.5 μm) exhibits subpicosecond carrier lifetime. This behavior is due to the presence of defects caused by the plasma particles during growth. To gain a better understanding of the nature of these defects, samples were annealed over the temperature range 500–700 °C and evaluated by variable energy positron annihilation measurements, transient ellipsometric surface photoreflectance, sheet resistance, and n–i–n device resistivity studies. For all samples, the resistivity increased with annealing, as did the carrier lifetime from the optical measurements which suggests the presence of more than one defect type in the material. Positron annihilation studies suggest that the open volume defects, present in the as-grown material as single vacancies and vacancy clusters, become larger upon annealing although this effect may be obscured in the Be-doped samples at higher annealing temperatures. The Be-doped samples were found to be more resistive and optically faster than the undoped material. Also, the sheet resistance measurements may indicate that the Be in the doped samples is activated at annealing temperatures above 600 °C, since the material changes from weakly n-type to p-type.