We have studied the folding kinetics of the core intermediate (I) state of RNase H by using a combination of single-molecule FRET (smFRET) and hidden Markov model analysis. To measure fast dynamics in thermal equilibrium as a function of the concentration of the denaturant GdmCl, a special FRET labeled variant, RNase H 60113, which is sensitive to folding of the protein core, was immobilized on PEGylated surfaces. Conformational transitions between the unfolded (U) state and the I state could be described by a two-state model within our experimental time resolution, with millisecond mean residence times. The I state population was always a minority species in the entire accessible range of denaturant concentrations. By introducing the measured free energy differences between the U and I states as constraints in global fits of the GdmCl dependence of FRET histograms of a differently labeled RNase H variant (RNase H 3135), we were able to reveal the free energy differences and, thus, population ratios of all three macroscopic state ensembles, U, I and F (folded state) as a function of denaturant concentration.