Excitation and relaxation mechanisms of Yb3+ions in InP and InP0.93As0.07alloy have been studied based on optically detected cyclotron resonance (ODCR), temperature dependent photoluminescence (PL), and PL decay measurements. It is shown with use of ODCR that charge carriers, most probably electrons, are captured by Yb3+ions. PL transients show that the decay rate of the Yb3+PL decreases with increasing As contents. Evidence is presented proving that the excited state of the Yb3+ion decays by transfer of energy to the band states of InP, while in the case of InPAs, the energy may also be transferred to an exciton bound at the Yb3+ion. ©1995 American Institute of Physics.