Electrical aging can increase non-radiative recombination processes in organic light-emitting diodes (OLEDs) which leads to a reduction in device efficiency. We investigated aging-related changes of the opto-electronic properties of OLEDs by a combination of photoluminescence, electroluminescence (EL), and photoluminescence excitation spectroscopy as well as measurements of the incident photon-to-current efficiency (IPCE). The EL measurements revealed a pronounced decrease in radiative recombination after electrical aging, and it is demonstrated here that IPCE measurements can make a useful contribution to identify the materials and processes that are affected most by electrical aging.