Protein–protein association generally occurs through the formation of an ensemble of transient encounter complexes that can dock into a final stereospecific complex. NMR paramagnetic relaxation enhancement (PRE) can detect lowly-populated states (< 1%), and has recently been applied to visualize an encounter complex ensemble during protein association. Here, we demonstrate that surface electrostatic potentials are key determinants for the encounter complex formation. We further show that productive on-pathway and nonproductive off-pathway encounters can be distinguished by rationally designed surface charge mutants. Perturbation of the on-pathway encounters results in reduced equilibrium binding affinity by altering the association kinetics. Measurement of PRE and binding thermodynamic combined with surface charge modulation can thus provide useful information on the heterogeneous nature of metastable encounter complexes.