Nine photo-activated acrylic-type (EPL) soft lining materials were produced experimentally to examine their durability. The purpose of this study was to investigate the effect of photo-irradiation on the hardness of experimental soft lining materials. The hardness (Hex) of the nine EPLs was increased 4.3-170.1% by exposing them to light. The effect of irradiation on the Hex of the EPLs was remarkable when the material contained much ethylmethacrylate, together with smaller amounts of di-n-butyl phthalate. Moreover, the Hex tended to increase for EPLs in which the liquid portion contained less di-n-butyl phthalate or more ethanol (Et). The weight change ratio (Wc) was greater for EPL specimens containing less Et. The Wcs ranged from 0.118 to 0.661% after 8 weeks. These results showed that the Hexs of six of the EPLs were approximately equal to those of four commercially available soft lining materials, and these six EPLs may have properties favouring their use in clinical dentistry.