The adhesion force between two 11-amino-1-undecanethiol self-assembled monolayers has been measured as a function of pH in solutions of two ionic strengths, high (10(-1) M) and low (10(-4) M). In high ionic strength solutions, the force titration curve approximates to a sigmoidal step, centered at ca, pH 3, with negligible adhesion at low pH and a high adhesion (20 nN) at higher pH but with a previously unobserved drop in adhesion almost to zero above pH 12. In low ionic strength solutions, the shape of the force titration curve is radically different, comprising two peaks centered at ca, pH 6 and 10. The length of the alkyl spacer has a strong effect on the magnitude of the peak at pH 6 which increases significantly with decreasing spacer length, whereas the peak at pH 10 is unaffected. We propose that the pH 6 peak in low ionic strength conditions and the very high adhesion in high ionic strength buffer are due to a hydrophobic interaction between two disordered monolayers and that in-plane hydrogen bonding between neutral and charged amine groups increases the surface ordering and reduces this effect in low ionic strength solutions. The drop in adhesion at high pH in both ionic strength solutions is unexpected. However, Fourier transform infrared data provide evidence for the presence of sulfur-containing groups in the monolayer surface, and the ionization of these groups at high pH resulting in electrostatic repulsion may explain the zero adhesion. In support of this hypothesis, a long-range, presumably electrostatic, repulsion is indeed observed in the approach part of the force-displacement curves.