We report on negative differential photovoltage (DPV), which is observed under modulated photoexcitation of a double heterojunction, when the common positive photovoltage increment due to photocurrent modulation is suppressed by high bias current. The negative DPV was shown to be due to the effect of photoconductivity on the series resistance of the heterojunction and due to the modulation of junction temperature. In AlGaInP double heterojunction light-emitting diodes, the magnitudes of negative DPV in the range of −10 μV correspond to the estimated variation of series resistance and junction temperature as low as ∼10−3 Ω and ∼10−2 K, respectively.