Measurements were made on the photosensitivity of vinyl cinnamate-vinyl acetate co-polymers which have various contents of vinyl cinnamate. The data obtained satisfy the theoretical equation S=kC² reported in the previous paper (ref. 2). The photosensitivity of a co-polymer containing 59.9 mol-% of vinyl cinnamate was increased by 5-nitroacenaphthene, a sensitizer, in the same degree (about×120) as that of poly (vinyl cinnamate) prepared by the esterification of poly (vinyl alcohol) in alkaline water (ref. 6). A reciprocity law failure was observed in the measurements of the photosensitivity by the method using a gray scale.