Upon light exposure, photosensitizers generate reactive chemical species that lead to cellular membrane alteration. On the one hand, this property is exploited in photodynamic therapy to irreversibly destroy diseased tissues. On the other hand, the reactivity of photosensitizers with lipid membranes prevents encapsulation or loading in liposomes for delivery applications. Polymersomes, which are vesicles made of amphiphilic polymers, have been used as drug carriers due to their superior robustness over liposomes. We have investigated the photo-response of prototypical polymersomes loaded with a classical chlorine photosensitizer. We have observed a complex sequence of light-induced morphological changes. Using micromechanical assays based on micropipette manipulation, we have quantitatively monitored the different phases of the photo-response, which include membrane area variation, osmotic swelling, membrane cross-linking and vesicle deflation. We have thus gained insight into the complex cascade of chemical reactions involved in photosensitization. Finally, our findings suggest that composite chlorine-copolymer vesicles may be used as a new class of light-sensitive drug carriers.