Device degradation of GaAs-based AlGaAs/GaInP/GaInAs ridge waveguide (RW) laser diodes is found to lead to peculiarities in the longitudinal mode spectrum. These features give information on the position of inhomogeneities along the laser stripe. Using monochromatic cathodoluminescence (CL), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX) these inhomogeneities are found to be located in the region of the quantum well (QW) and to be caused by interdiffusion of the lattice constituents. This interdiffusion extends over the active region from the interfaces of the GaInP waveguides to the GaAs spacer layers. It is triggered by nonradiative recombination under laser operation.