Analysis of the thermodynamics of the interactions between the D3 T-cell receptor (TCR) and its natural ligand, an HIV peptide bound to a HLA-A*0201 (HLA-A2) major histocompatibility complex (MHC) protein, shows both similarities and striking differences when compared with the 2B4 TCR binding to its peptide−MHC ligand. The equilibrium thermodynamic parameters of both reactions are consistent with a conformational adjustment at the binding interface during the formation of specific TCR−peptide−MHC complexes. However, osmolytic reagents that dehydrate protein surfaces have profoundly different effects on the strength of the two reactions, indicating that water molecules make very different contributionsenhancing the binding of D3 TCR but weakening the binding of 2B4 TCR. The use of these different mechanisms by TCRs to recognize ligands might be an important means augmenting their inherent cross-reactivity.