© 2018 The Japan Society of Applied Physics. The generation of acetic acid from the ethylene vinyl acetate (EVA) encapsulant in crystalline Si photovoltaic (PV) modules was investigated by free-volume analysis with positron annihilation lifetime spectroscopy (PALS). The reduction in size of a free-volume hole attributable to deacetylation was clearly observed near the surface of the EVA encapsulant for a module aged in a damp heat (DH) test following UV irradiation (hereafter referred to as the post-UV DH test). Deacetylation in the post-UV DH test was considerable compared with that in a normal DH test. The depth profile of the size of a free-volume hole in a sample exposed outdoors was revealed to be very similar to that obtained in the post-UV DH test. This strongly implies that deacetylation was promoted near the interface between EVA and other components in the accelerated degradation test as well as by outdoor exposure.