The thermodynamics of dissociation of vancomycin and ristocetin dimers in the presence and absence of specific ligands has been studied by direct microcalorimetry over a range of temperature, pH and ionic strength conditions in H 2 O and D 2 O. Dimerization of these antibiotics is exothermic with large temperature dependence (ΔC p ) and consequent entropy–enthalpy compensation effects that may be consistent with solvation changes associated with burial of non-polar surfaces during macromolecular association. For vancomycin, no significant ionic strength effects are observed, so non-specific electrostatic contributions are probably discounted, but pH and buffer effects on the thermodynamic parameters are consistent with hydrogen ion uptake and pK shift in the dimerization process. Vancomycin dimerization is significantly enhanced in the presence ofspecifically binding ligands: acetate, N-acetyl-D-Ala, and N α ,N ε -diacetyl-L ys-D-Ala-D-Ala, in increasing order of effectiveness.The dipeptide ligand N-acetyl-D-Ala-D-Ala promotes higher oligomerization and crystallization of the complex. Ristocetin, in contrast, displays no such ligand effects; it shows a slight reduction in dimerization in the presence of strongly binding N α ,N ε -diacetyl-L ys-D-Ala-D-Ala. This difference may reflect the need for flexibility in the antibiotic structure to allow ligand-induced aggregation. Eremomycin dimerizes strongly even in the absence of ligand. Dissociation of the vancomycin–N α ,N ε -diacetyl-Lys-D-Ala-D-Ala dimer complex is slow (k diss ca. 0.005 s -1 ) and kinetics can be measured by conventional UV difference techniques.