G protein-coupled receptors (GPCRs) constitute the largest family of integral membrane proteins present in all eukaryotic cells, yet relatively little information about their structure, folding, and stability has been published. In this work, we describe several approaches to characterizing the conformational stability of the human adenosine A(2)a receptor (hA(2)aR). Thermal denaturation and chemical denaturation were not reversible, yet clear differences in the unfolding behavior were observed upon ligand binding via circular dichroism and fluorescence spectrometry. We found that the stability of hA(2)aR was increased upon incubation with the agonist N(6)-cyclohexyladenosine or the antagonist theophylline. When extracellular disulfide bonds were reduced with a chemical reducing agent, the ligand binding activity decreased by ~40%, but reduction of these bonds did not compromise the unfolding transition observed via urea denaturation. Overall, these approaches offer a general strategy for characterizing the effect of surfactant and ligand effects on the stability of GPCRs.