A systematic study of the effects of Ti doping on the electrical and optical properties of GaAs and InP has been carried out employing both melt and solution‐grown crystals. Utilizing deep level transient spectroscopy, Hall effect measurements, photoconductivity, and optical absorption measurements, it was found that Ti introduces two deep levels in GaAs at Ec -0.23 eV and Ec -1.00 eV which were identified as the Ti3+/Ti2+ acceptor level and the Ti4+/Ti3+ donor level, respectively. In InP the Ti4+/Ti3+ donor level was found near midgap at Ec -0.63 eV, while the Ti3+/Ti2+ acceptor level was found to lie within the conduction band. As a consequence of the midgap position of this donor level, we developed a formulation for producing semi‐insulating InP based on doping with Ti to compensate shallow acceptors. Resistivities in excess of 107 Ω cm can easily be obtained using this technique. This is the first semi‐insulating III‐V compound having a compensation mechanism based on a deep donor impurity. In view of the fact that Ti is expected to have a very low diffusivity in InP, Ti‐doped semi‐insulating InP should exhibit far greater thermal stability than Fe‐doped InP and thus it should prove technologically significant.