The spectral electroluminescence of a monolithic triple-junction solar cell reveals the sub-cell open-circuit voltages under variation of temperature and carrier concentration. We present an electroluminescence set-up that is able to acquire a full spectrum in less than 10 s and give insight into the voltage-extraction process. The sum of the sub-cell voltages is in excellent agreement to open-circuit voltages acquired under concentrated illumination. The temperature-induced voltage losses are investigated and explained by a bandgap dependent increase in intrinsic carrier concentration. Finally, the accurate extraction of the sub-cell bandgaps helps to identify a temperature rise with increasing current density. Copyright © 2013 John Wiley & Sons, Ltd.