In this study the influence of water concentration in self-etching (SE) primers on their aggressiveness and bond strength to ground enamel was investigated. Five experimental primers with 0, 5, 10, 20, or 40wt% of water were formulated, using Clearfil SE Bond (R) (CSEB) as a commercial reference. Primers were applied to bovine incisors following application of bonding resin and composite restoration. Bond strength was evaluated through shear testing and data were statistically analyzed (5%). The etching aggressiveness was evaluated by SEM. Water concentration significantly affected bond strength and etching aggressiveness. The highest bond strength value (MPa) was found for the primer containing 20% of water (24.7), followed by CSEB (23.8). Groups containing 5% (20.5) and 10% (20.2) of water showed an intermediate performance, while the lowest values were observed for the 0% (17.3) and 40% (16.7) groups. SEM analysis showed increasing aggressiveness for increased water content.