This article presents a novel scheme for condition monitoring of dc-link capacitors in modular multilevel converters (MMCs). The proposed solution uses estimated capacitance values of the dc-link capacitors for indicating their state-of-health (SoH). Moreover, a comparative approach is proposed where the estimated capacitance of all submodule capacitors are used to separate parameter drifts caused by aging from parameter drifts caused by other factors such as temperature change. It is shown in simulation and experimental results that a short-term equal drift in all capacitance estimates can be a result of factors other than aging. However, a drift in the capacitance of one capacitor compared to the average capacitance of all submodules may be attributed to aging of that specific unit. Using the proposed comparative technique, there is no need for additional temperature sensors to incorporate the effect of temperature variations on the online estimations. Simulation and experimental results prove an overall estimation error of less than 1% when applying the proposed comparative technique.