The reversibility of the unfolding‐refolding transition of horse muscle phosphoglycerate kinase, induced by guanidine hydrochloride (Gdn · HCl), was studied using the regain of enzyme activity as a probe of the native structure. An irreversibility in the reactivation process was detected when the protein was incubated in a critical concentration of denaturant (0.7 ± 0.1 M Gdn · HCl). This apparent irreversibility was observed for the unfolding process (N → D) as well as for the refolding process (D → N). The formation of the trough followed biphasic kinetics at 23°C, the first phase obeying a first‐order reaction corresponded to an isomerization of an intermediate; the second phase, protein‐concentration‐dependent, was suppressed by lowering the temperature to 4°C. The structural properties of the inactive species were studied; all the β structures were recovered, but about 29% of the helical structures remained unfolded, and two SH groups were buried. Simulated kinetics were compared with the experimental results and were used to extend the minimum folding scheme previously proposed from equilibrium and kinetic studies [Betton et al. (1984) Biochemistry 23, 6654–6661; Betton et al. (1985) Biochemistry 24, 4570–4577]. The intermediates trapped under these conditions were structured but devoid of catalytic activity. Taking into account the structural properties of these species, the nature of the interactions involved in their formation and stabilization is discussed.