Sodium hypochlorite is one of the disinfectants recommended for impressions, usually in a 1:10 dilution to minimize the negative effects on the impression material and poured gypsum cast. Dilution causes a loss of antimicrobial properties, however. Sodium hypochlorite is known to have enhanced antimicrobial properties at lower pH levels. This study evaluated the antimicrobial activity of sodium hypochlorite at progressively lower pH levels. A metal model of a dental arch was contaminated, and irreversible hydrocolloid impressions were made, cultured, disinfected (by immersion), and cultured again. Predisinfection and postdisinfection bacterial counts were compared. It was found that a 10-minute immersion in solutions reduced to pHs 7 to 11 consistently produced a 4-log (99.99%) or greater reduction in viable organisms. A pH of 10 was the only level that was consistently effective at decreased immersion times. It was effective in times of 3 minutes or greater.