We have used differential scanning calorimetry to determine the effect of low concentrations (C = 0–2 M) of the osmolyte sarcosine on the Gibbs energy changes (ΔG) for the unfolding of hen-egg-white lysozyme, ribonuclease A, and ubiquitin, under the same buffer and pH conditions. We have also computed this effect on the basis of the additivity assumption and using published values of the transfer Gibbs energies for the amino acid side chains and the peptide backbone unit. The values thus predicted for the slope ∂ΔG/∂C agree with the experimental ones, but only if the unfolded state is assumed to be compact (that is, if the accessibility to solvent of the unfolded state is modeled using segments excised from native structures). The additivity-based calculations predict similar ∂ΔG/∂C values for the three proteins studied. We point out that, to the extent that this approximate constancy of ∂ΔG/∂C holds, osmolyte-induced increases in denaturation temperature will be larger for proteins with low unfolding enthalpy (small proteins that bury a large proportion of apolar surface). The experimental results reported here are consistent with this hypothesis.