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Dual-junction solar cells have the potential for higher conversion efficiencies than single junctions, thanks to a better utilization of the solar spectrum. However, their optimization is more complex because the monolithic interconnection makes the individual subcell electrical measurement difficult or impossible. This study presents a new technique to measure the open-circuit voltage VOC of each individual subcell of a dual-junction device with a method that circumvents the need for completely selective illumination. The method is based on the description of the total device VOC as a function of two independent variables determined by two distinct illumination spectra. Without completely selective illumination, one can measure only a limited range of this function; however, an unambiguous extension gives values corresponding to the conditions of ideal selectivity, which correspond in turn to the VOC of the individual subcells at 1-sun illumination. This method does not rely on any specific electrical or optical model of solar cell. Method is successfully tested with a simple measurement setup based on commercially available luminescence diodes and voltmeters in combination with specially prepared three-terminal dual-junction thin-film silicon solar cells.