We study the critical behavior of 1.3&mgr;mInP-based strained-layer (SL) multiple-quantum-well (MQW) lasers at elevated temperatures. We show that, under the critical injection condition where the carrier density in the quantum wells reaches the maximum possible without causing any extra pile-up of carriers in the separate heterostructure confinement regions, an InP-based SL-MQW system exhibits an absorption-to-gain phase transition at some critical temperature(Tc). The characteristic feature of this phase transition shows excellent agreement with Landau theory of second-order phase transitions. It is demonstrated thatTcis a significant and meaningful quantity not only for laser design but also for characterizing the nature of an InP-based SL-MQW system in terms of condensed matter physics. ©1997 American Institute of Physics.