We previously showed that membrane-depolarizing agents such as K+ and ATP-sensitive potassium (KATP) channel inhibitors potentiate tumor necrosis factor-related apoptosis‑inducing ligand (TRAIL)-induced apoptosis in human melanoma cells, but not in normal melanocytes. In this study, we investigated whether the tumor-selective effect of depolarization was observed among different tumor cell types and the mechanisms by which depolarization potentiates death pathways. We found that K+ and KATP channel inhibitors elicited similar apoptosis-potentiating effects in human tumor cells with different origins, including leukemia, melanoma and lung cancer cells. In contrast, minimal potentiation of apoptosis was observed in non-transformed lung cells. The potentiation was associated with increased mitochondrial and endoplasmic reticulum stress death pathways. Upregulation of surface TRAIL receptor-2 expression and modulation of the caspase-3 activation pathway seemed to play roles in the enhancement of death signaling. Moreover, the results showed that depolarization and mitochondria‑derived reactive oxygen species (mROS) mutually regulated one another. Depolarization potentiated TRAIL-induced mROS accumulation. Conversely, scavenging of mROS by the antioxidant MnTBaP reduced depolarization, whereas mROS accumulation caused by metabolic inhibitors potentiated the depolarization. These findings suggest a positive loop between depolarization and mROS accumulation. This may provide a rationale for the tumor-selective cytotoxicity and/or potentiation of TRAIL cytotoxicity of a wide variety of ROS-producing substances in different types of tumor cells.