Apparent and partial molar enthalpies at 298 K of the aqueous solutions of cationic gemini surfactants 1,1′-didodecyl-2,2′-trimethylenebispyridinium dichloride (12-Py(2)-3-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-tetramethylenebispyridinium dichloride (12-Py(2)-4-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-octamethylenebispyridinium dichloride (12-Py(2)-8-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-dodecamethylenebispyridinium dichloride (12-Py(2)-12-(2)Py-12 Cl) were measured as a function of concentration and are here reported for the first time. The curve of the compound with the four carbon atoms spacer lies between those of the compound with spacer of 2 and 3 carbon atoms, not below the latter, as expected. This behaviour, we have already found for the same compounds here studied, but having methanesulfonate as counterion, is interpreted as an evidence of a conformation change of the molecule determined by stacking interactions between the two pyridinium rings, appearing at an optimum length of the spacer. The curves of apparent and molar enthalpies vs. concentration and surface tension measurements, here reported, show that this behaviour is independent on the counterion. On the contrary, the group contribution additivity for the counterion is respected, independently on the spacer length.Keywords. Apparent and partial molar enthalpies; cationic gemini surfactants; 1,1′-didodecyl-2,2′-trimethylenebispyridinium dichloride; 1,1′-didodecyl-2,2′-tetramethylenebispyridinium dichloride; 1,1′-didodecyl-2,2′-octamethylenebispyridinium dichloride; 1,1′-didodecyl-2,2′-dodecamethylenebispyridinium dichloride; micellization enthalpy, group contribution.Briefs. Apparent and partial molar enthalpies at 298 K of the aqueous solutions of cationic gemini surfactants 1,1′-didodecyl-2,2′-trimethylenebispyridinium dichloride (12-Py(2)-3-(2)Py-12 Cl);1,1′-didodecyl-2,2′-tetramethylenebispyridinium dichloride (12-Py(2)-4-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-octamethylenebispyridinium dichloride (12-Py(2)-8-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-dodecamethylenebispyridinium dichloride (12-Py(2)-12-(2)Py-12 Cl) have been measured as a function of concentration, in order to confirm the lack of methylene group additivity with respect to the enthalpic properties of the solutions, when it is added in the spacer, already found in the case of methansulfonates. The data obtained support the hypothesis of a conformation change dependent on the spacer length, but independent on the counterion.