We have determined the pK(a) of surface-bound primary amine groups by determining the surface potential as a function of solution pH from the magnitude of the electric double-layer force. Using colloid-probe atomic force microscopy (AFM), we measured the force as a function of separation between a particle of radius R = 10 mu m and a planar surface, each coated with a self-assembled monolayer of HS(CH2)(2)CONH((CH2)(2)O)(8)(CH2)(2)NH2. The force was measured from pH 3 to 7, and the surface potential was determined by fitting the results to solutions of the nonlinear Poisson-Boltzmann equation. The surface pK(a) of the primary amine group was found to be 5.0 +/- 0.2, in agreement with the results of contact-angle and chemical-force titrations on similar surfaces with primary amine groups. The surface charge density indicates that less than 1% of the NH2 groups are dissociated at pH 3, suggesting that ionization is very unfavorable in the local environment of the ethylene oxide chains. This noncontact method should be of general applicability to surfaces with ionizable groups and avoids the possible complications of large contact forces on the surfaces under study.