The dissociation constants of the conjugate acids of six cyclic diamines [piperazine, 1-methylpiperazine, 2-methylpiperazine, 1-ethylpiperazine, 1-(2-hydroxyethyl)piperazine, and 1,4-dimethylpiperazine] were calculated using the potentiometric titration method at (298, 303, 313, and 323) K. The pK(a) values of piperazine were compared with published data to validate the procedure used. The thermodynamic quantities (Delta H degrees and Delta S degrees) for the dissociation processes were determined using the van't Hoff equation. A trend is proposed related to the variation of the pK(a) with the addition of different radical groups to the base piperazine molecule.