Kinetic and equilibrium constants for lactose repressor-operator DNA interaction have been examined as a function of salt concentration, size and sequence context of the operator DNA, and temperature. Significant salt effects were observed on kinetic and equilibrium parameters for pLA 322-8, an operator-containing derivative of pBR 322, and pIQ, an operator and pseudooperator-containing derivative of pBR 322. The association rate constant and equilibrium constant for the 40 base pair operator fragment were also salt dependent. Data for all the DNAs were consistent with a sliding mechanism for repressor-operator association/dissociation [Berg, O. G., & Blomberg, C. (1978) Biophys. Chem. 8, 271-280]. Calculation of the number of ionic interactions based on salt dependence yielded a value of approximately 8 for repressor binding to pIQ and pLA 322-8 vs. approximately 6 for the repressor-40 base pair fragment. These data and the differences in binding parameters for the plasmids vs. the 40 base pair operator are consistent with the formation of an intramolecular ternary complex in the plasmid DNAs. Unusual biphasic temperature dependence was observed in the equilibrium and dissociation rate constants for pLA 322-8, pIQ, and the 40 base pair fragment. These observations coupled with a discontinuity found in the inducer association rate constant as a function of temperature suggest a structural change in the protein. The large positive entropy contributions associated with repressor binding to all the DNAs examined provide the significant driving force for the reaction and are consistent with involvement of ionic and apolar interactions in complex formation.