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The DeltaG degrees (N)(-)(D) value obtained from extrapolation to zero denaturant concentration by the linear extrapolation method (LEM) is commonly interpreted to represent the Gibbs energy difference between native (N) and denatured (D) ensembles at the limit of zero denaturant concentration. For DeltaG degrees (N)(-)(D) to be interpreted solely in terms of N and D, as is common practice, it must be shown to be independent of denaturant concentration. Because DeltaG degrees (N)(-)(D) is often observed to be dependent on the nature of the denaturant, it is necessary to determine the circumstances under which DeltaG degrees (N)(-)(D) can be interpreted as a property solely of the protein. Here, we use proton inventory, a thermodynamic property of both the native and denatured ensembles, to monitor the thermodynamic character of denaturant-dependent aspects of N and D ensembles and the N right arrow over left arrow D transition. Use of a thermodynamic rather than a spectral parameter to monitor denaturation provides insight into the manner in which denaturant affects the meaning of DeltaG degrees (N)(-)(D) and the nature of the N right arrow over left arrow D transition. Three classes of proteins are defined in terms of the thermodynamic behaviors of their N right arrow over left arrow D transition and N and D ensembles. With guanidine hydrochloride as a denaturant, the classification of protein denaturations by these procedures determines when the LEM gives readily interpretable DeltaG degrees (N)(-)(D) values with this denaturant and when it does not.