The isolated N-terminal 1-69 domain of the 434-phage repressor, R69, and its covalently linked (head-to-tail and tail-to-tail) dimers have been studied by differential scanning microcalorimetry (DSC) and CD. At neutral solvent conditions the R69 domain maintains its native structure, both in isolated form and within the dimers. The stability of the domain depends highly upon pH within the acidic range, thus at pH 2 and low ionic strength R69 is already partially unfolded at room temperature. The thermodynamic parameters of unfolding calculated from the DSC data are typical for small globular proteins. At neutral pH and moderate ionic strength, the domains of the dimers behave as two independent units with unfolding parameters similar to those of the isolated domain, which means that linking two R69 domains, either by a long peptide linker or by a designed C-terminal disulfide bridge, does not induce any cooperation between them.