Aging properties of AlGaAs high power laser arrays were monitored by photocurrent (PC) measurements. We demonstrate that this method allows a quantitative analysis of the aging process and provides insight into the microscopic degradation mechanisms. In graded index separate confinement heterostructures, the PC spectra in the spectral range of the laser transition display a substantial current decrease with aging. This is attributed to enhanced rates of surface recombination and defect generation in the graded gap region. In contrast, step index structures exhibit a much smaller change of PC. The PC results are complemented by measurements of low-frequency laser current noise and front facet temperature. ©1997 American Institute of Physics.