1 MeV Fe+ was implanted into n-type InP and InGaAs layers at different substrate temperatures, -196degreesC, room temperature (RT), 100degreesC and 200degreesC to obtain high-resistivity regions. The sheet resistivity of the InP and InGaAs epilayers; grown on semi-insulating (SI) InP substrates was measured as a function of substrate temperature and post-implantation annealing temperature (100 - 800degreesC). For InP, a maximum sheet resistivity of similar to1x10(7) Omega/rectangle was achieved for samples implanted at -196degreesC, RT and 100degreesC after annealing at 400degreesC. For InGaAs samples, a maximum sheet resistivity of 1x10(7) and 2.3x10(6) Omega/rectangle is obtained for -196degreesC and RT implants respectively after annealing at 650degreesC for 60s. In both InP and InGaAs, the isolated regions exhibit good stability to heat treatment up to 500degreesC for all cases irrespective of the irradiation temperature. The iron depth profile obtained by secondary ion mass spectrometry (SIMS) shows that iron does not diffuse up to an annealing temperature of 550degreesC in both InP and InGaAs for all implantation temperatures. These results are novel since high sheet resistivity (similar to5x10(6) Omega/rectangle) is obtained in both InP and InGaAs samples implanted at -196degrees and RT, and annealed at 400degreesC. These data demonstrate the potential usefulness of iron implantation for isolation of InP/InGaAs based devices.