The protein-ligand binding interactions in solution were investigated using mass spectrometry. The H/D exchange and mass spectrometry-based technique termed as stability of unpurified proteins by rates of H/D exchange (SUPREX) was developed to evaluate thermodynamic stability of proteins. The analysis was initiated by 10-fold dilution of stock solution of protein in series of deuterated exchange buffers which varied in concentration of deuterated guanidine. The results show that increased H/D time shifted the SUPREX curve transition midpoint to lower denaturant.