Hydroxyapatite (HAp) deposition onto positively charged surfaces (i.e., self-assembled monolayers; (SAMs) terminated with NH2 head groups) and negatively charged surfaces (i.e., OH-SAMs (weak) and COOH-SAMs (strong)) soaked at 50degreesC in aqueous supersaturated solutions (1.5 SBF, pH 7.0-7.6; SBF = simulated body fluid) was investigated using a quartz-crystal microbalance. The results revealed that the solution conditions greatly influenced the formation of HAp on the charged surfaces. In a stable supersaturated solution of simulated body fluid (1.5 SBF, pH <7.4), more strongly negative surfaces had a more powerful induction capability for the heterogeneous nucleation of HAp (COOH > OH), whereas nucleation was obviously prohibited on a positive surface (NH2-SAM). On the other hand, after the calcium phosphate particles had nucleated homogeneously in an unstable soaking solution (1.5 SBF, pH greater than or equal to7.4), adhesion of the HAp microparticles to the NH2-SAM was observed. A two-step interaction is conceivable to describe the formation of HAp on the positive NH2-SAM: At the first stage, electrostatic interaction dominates the adhesion of HAp microparticles; at the second stage, hydrogen bonds possibly form between the HAp microparticles and the amino head groups of the NH2-SAM, for a firm bonding with the substrate, and the microparticles grow progressively into a thin film. The electrophoretic behaviors of the HAp microparticles confirmed this hypothesis.