Monte Carlo simulations were utilized to investigate the formation of dimers involving urea and N-methylacetamide (NMA) inaqueous solution. The dimers-show much variation in structure, including both singly and doubly hydrogen-bonded as well as stacked configurations, depending on their constitution and intermolecular separation. Relative stabilities of the dimers were found to be urea dimer > mixed dimer > NMA dimer. These results suggest that protein denaturation in the presence of urea involves the participation of aggregates of the denaturant.