The osmotic lysis of photodamaged lysosomes is a critical event for killing tumor cells. How the photodamage increases lysosomal osmotic sensitivity is still unclear. In this work, the effect of the photooxidation of membrane thiol groups on the lysosomal osmotic sensitivity was studied by measuring the thiol groups with 5,5′-dithio-bis(2-nitrobenzoic acid) and examining the lysosomal β-hexosaminidase latency loss in a hypotonic sucrose medium. The results show that methylene blue–mediated photooxidation of lysosomes decreased their membrane thiol groups and produced cross-linkage of membrane proteins (molecular weight ranging from 75 000 to 125 000), which was visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Simultaneously, the lysosomal osmotic sensitivity increased. These photoinduced alterations of the lysosomes could be recovered by reducing the oxidized thiol groups with dithiothreitol. It indicates that the photooxidation of membrane thiol groups can increase the lysosomal osmotic sensitivity and therefore provides a new explanation for the photoinduced lysosomal lysis.