Spiropyrans undergo reversible photoisomerization between their ring-closed (spiro) and ring-open (merocyanine) forms when exposed alternately to ultraviolet and visible light. Rates of K+ ion leakage from phosphatidylcholine unilamellar vesicles containing a K+-selective crown ether attached to an amphiphilic spiropyran increased markedly when the vesicle assembly was illuminated with IN light. This effect was fully reversible, with K+ leak rates returning to their basal levels upon illumination with visible light. In contrast, UV-induced photoisomerization of a similar spiropyran-crown construct that did not strongly coordinate K+ was not accompanied by enhancement of K+ leakage. Transient spectrophotometry revealed that, immediately following photoisomerization, the merocyanine form of the dye was in a moderately polar environment, consistent with its location in the glycerophosphate backbone region of the vesicle. Within several milliseconds, the polarity of the environment increased, as indicated by a hypsochromic shift in the merocyanine visible absorption band. This environmental relaxation within the vesicle is similar to the behavior of simple vesicle-bound spiro compounds that lack an appended crown ether and has been attributed to relocation of the dye toward the aqueous-organic interfacial region of the membrane following its conversion to the more polar merocyanine form. The enhanced K+ leak rate attending photoisomerization is therefore suggested to be a consequence of this relocation, thereby giving the tethered crown ether proximity to K+ ions located in the aqueous core and bulk aqueous phases. Following either photoinitiated or thermal ring closing, the dye-crown construct again becomes confined to the membrane interior, terminating merocyanine-mediated K+ transport. The transport quantum yield, defined as the number of K+ ions released per photon absorbed, is similar to0.3.