The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition AH; in poly(L-ornithine) in 0.1 N KCl has been determined calorimetrically to be -1530 f 210 and -1270 f 530 cal/mol a t loo and 25OC, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute AH;, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH-ion, and the heat of ionization of the &amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D,L-ornithine). Since it was discovered that poly(L-ornithine) undergoes chain cleavage a t high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix-coil transition curve for poly(L-ornithine) a t pH 11.68 in 0.1 N KCl in the Oo400C region, the transition temperature T,, and the quantity ( d O h / d T ) ~~~ have been obtained. From these values, together with the measured values of AH;, the changes in the standard free energy AG; and entropy AS;, associated with the uncharged coil-to-helix transition a t 10°C have been calculated to be -33 cal/mol and -5.3 cal/mol deg, respectively. The value of the Zimm-Bragg helix-coil stability constant u has been calculated to be 1.4 X and the value of s has been calculated to be 1.06 a t 10OC. and between 0.60 and 0.92 a t 25OC.