The net gain per unit length ( ) versus current ( ) is measured at various temperatures for 1.3 μm InGaAsP-InP double heterostructure lasers. is found to vary linearly with the current at a given temperature. The gain bandwidth is found to decrease with decreasing temperature. The lasing photon energy decreases at 0.325 meV/K with increasing temperature. Also, the slope at the lasing photon energies decreases with increasing temperature. This decrease is more rapid for K. This faster decrease is consistent with the observed higher temperature dependence of threshold (low T0at high temperatures) of 1.3 μm InGaAsP lasers. A carrier loss mechanism, due to Auger recombination, also predicts that should decrease much faster with increasing temperature at high temperatures. We also find that the slope decreases slowly with increasing temperature for a GaAs laser, which is consistent with the observed temperature dependence of threshold of these lasers.