The recombinant humanized antibody (rhuMAb) VEGF has a high affinity for vascular endothelial growth factor and is currently being evaluated in clinical trials as a cancer therapeutic. Under acidic pH and low ionic strength conditions, the antibody was predominantly present as monomer. Under physiological conditions, the appearance of significant amounts of a noncovalent, reversible dimer were observed by size-exclusion chromatography. The kinetics and thermodynamics of the reversible self-association for rhuMAb VEGF monomer were investigated as a function of pH, temperature, and ionic strength by size-exclusion chromatography using the concentration jump method. The rate constant for dimer formation ranged 23-112 M(-)(1) min(-)(1) under the conditions studied, values that are significantly lower than those reported in the literature for other proteins that self-associate. The rate constant for dissociation ranged 0.0039-0.021 min(-)(1). Gibbs' free energies, enthalpies, entropies, and activation energies were determined and revealed that dimer formation is optimal at pH 7.5-8.0, which may be reflective of charge shielding occurring near the pI of the protein. There was a negative change in entropy for dissociation (values from -18.1 to -12.8 cal/mol K). In the presence of D(2)O or 1 M NaCl, dimerization was enhanced. The results of the kinetic and thermodynamic analysis of this study indicate that rhuMAb VEGF dimerization occurs primarily through hydrophobic interactions.