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It has been suggested that the long‐term degradation mode in (Al,Ga)As double‐heterostructure lasers is caused by the growth of an optically thick reflectivity‐lowering oxide formation on the emissive portion of the mirror facets. We find that the long‐term degradation behavior of our superior lasers cannot be explained by this approach. While not ruling out all conceivable models for the oxide growth rate, our conclusions based upon two simple and physically plausible models corroborate our separate experimental study which found that facet oxide formation is not directly relevant to laser reliability in the regime characterized by cw operation in a 70 °C dry‐nitrogen ambient and outputs of ∼5 mW/facet for stripe widths equal to 12 μ and lifetimes ?30 000 h. We have also examined a commonly used expression for the external differential quantum efficiency and find that it has historically been misinterpreted by many workers.