The improved thermal stability of 1.5 mu m InGaAlAs- compared with InGaAs-based lasers is investigated using a combination of low temperature and high pressure techniques. The results indicate that this is due to lower nonradiative Auger recombination in the InGaAlAs devices because of the higher conduction band offset made possible with the InGaAlAs system which results in a lower hole density in the quantum wells at threshold.