Abstract Light intensity modulated photocurrent and photovoltage spectroscopies, IMPS and IMVS respectively, are characterization techniques for studying charge carrier transport and recombination properties of photosensitive samples such as photovoltaic solar cells. In these techniques controlling the modulated light flux is key to obtaining accurate results. Typically, the electroluminescence of the light source is considered frequency-independent and therefore, it may be estimated from the modulated current delivered by the power source. However, some anomalies may appear when the experimental requirements demand large variations in the measurement conditions. Herein, an analysis is presented on the unusual low-frequency response of IMPS and IMVS which appears for some light sources at high illumination intensities. We found that a frequency-dependent modulation of the light source electroluminescence should be accounted for, rather than the traditional steady-state calibration of the setup, as it may affect the accuracy and even produce undesired artifacts during the measurements. A protocol for detecting the modulation of the electroluminescence is proposed, combining the simultaneous use of the IMPS of a reference photodiode and the impedance spectroscopy of the light source. Discerning whether these low-frequency signal ‘tails’ are due to the measurement setup or the sample is of major importance to avoid misinterpretations in any study. This is particularly important for preventing misinterpretations in studies on perovskite solar cells whose instability and ion-conductivity phenomena relate to the low-frequency region of the spectra.