The effect of the pH value of single-bottle, self-etching 2-hydroxyethyl methacrylate (HEMA)-free adhesives on their shelf lives with bonding ability was investigated. Two experimental adhesives [SA-1 (pH=3.0); SA-2 (pH=2.1)] comprising 6-methacryloyloxyhexyl phosphonoacetate (6-MHPA), 4-acryloyloxyethyltrimellitic acid (4-AET), Bis-GMA, water, and acetone, in which the pH values were controlled with 1N NaOH, were stored for 6 weeks at 50 degrees C in air. Shear bond strengths of aged adhesives to ground and unetched enamel and dentin were measured at a crosshead speed of 1.0 mm/min, and the data were analyzed by ANOVA. The resin-tooth bonding interfaces were further observed with SEM. With SA-1, the bond strengths to the tooth substrates were maintained throughout the 6 weeks. However, with SA-2, bond strength significantly degraded from weeks 4 to 6 (p<0.05). Based on SEM observation, non-aged and 6-week-aged adhesive SA-1 appeared to be strongly adhered to both enamel and dentin. It was found that the stability of the self-etching adhesive was influenced by its pH value.