A 33-residue pseudo-wild-type GCN4 leucine zipper peptide is used to probe the equilibrium conformational population in proteins. 13C(α)-NMR shows that chain sites differ in structural content at a given temperature, and that two dimeric folded forms are evident at many sites. Spin inversion transfer experiments are reported bearing on the thermodynamics and kinetics of interconversion of the two dimeric folded forms (F(a) ⇆ F(b)) at the 13C(α)-labeled position L13. At each temperature, at conditions wherein the population of unfolded chains is quite small, inversion of the F(a) spins via a tuned Gaussian π-pulse is followed by a time interval (τ), interrogation, and recording of the free induction decay. Fifteen such inversions, with varying τ provide the time course for recovery of equilibrium magnetization after inversion. Similar experiments follow inversion of the F(b) spins. Re-equilibration is known to be modulated by four first-order rate constants: two (T(1a)-1) and T(1b)-1) for spin-lattice relaxation intrinsic to the respective sites, and two (k(ab) and k(ba)) for the conformational change. All four follow from joint, Bayesian analysis of all the data at each temperature. The equilibrium constant at each temperature for this local transition, determined simply from the equilibrium relative magnetizations at F(a) and F(b) sites, agrees well with the kinetic ratio k(ab)/k(ba). The standard Gibbs energies, enthalpy, and entropy follow. Activation parameters, both ways, are accessible from the rate constants and suggest a transition state with high Gibbs energy and enthalpy, but with entropy between those of F(a) and F(b).