This paper reports experimental results on the impact of the temperature and spectral conditions on the behaviour of III-V multi-junction solar cells, employed in concentrator photovoltaic modules, operating under ultra-high concentration (UHC) conditions. The electrical parameters of the I–V curves of a 3-junction solar cell were measured using an experimental set-up based on a multi-flash indoor solar simulator, which allows adjustment of the cell operating temperature up to 85°C and reaching irradiance concentration levels up to 2200 suns under controlled spectral conditions. The measurements presented are unprecedented, as there are no previous publications that combine different spectral and temperature-controlled experimental conditions in the UHC range, thus extending the range of studies reported in works carried out at lower concentrations.