Thermodynamic parameters for self-association of theophylline in aqueous solution adjusted to ionic strength 0.20 were determined by proton unclear magnetic resonance spectroscopy. The results obtained at pD 5.8 were as follows : ΔG°=-0.95kcal/mol at 30°C (K=4.8M^-1), ΔH°=-6.3kcal/mol, and ΔS°=-18e.u. It was found that the driving force for self-association involves electrostatic, polarization, and dispersion interactions, and the contribution of hydrophobic bonding is small. A possible mode of self-association of theophylline is postulated. It was found that self-association of theophylline occurs more easily in the neutral state than in the ionized state.