This study investigated the effects of various immersion times and concentrations of sodium hypochlorite on irreversible hydrocolloid impressions and resultant gypsum casts. Irreversible hydrocolloid impressions of a test die were immersed for 1, 5, or 10 minutes in water (control), 5.25%, 0.525%, and 0.0525% sodium hypochlorite and then cast in a Type III stone and a Type V stone. Each stone specimen was evaluated for detail reproduction, dimensional change, surface roughness, and surface hardness. The results indicated that impressions may be immersed in sodium hypochlorite for any of the experimental times and concentrations without negative effects on Type V stone casts. However, immersion of impressions in 5.25% sodium hypochlorite causes some surface deterioration on Type III stone casts.