Flavor enhancing activities synergistic with monosodium L-glutamate and stimulus thresholds of ribonucleotide derivatives have been correlated with their chemical structures following Hansch-Fujita's method. Regression analyses were made separately for two groups, 2-substituted inosine 5'-phosphates and S-substituted 2-mercaptoinosine 5'-phosphates. Superdelocalizability, net π-charge, Hammett σm, and hydrophobic parameter π were used in the analyses. Nuclear magnetic resonance chemical shift of the-SCH2X group and a dummy variable D, which represents a proton accepting oxygen atom at the γ- or δ-position from the sulfur atom in substituents attached to the 2-position of purine skeleton, were employed successfully in a series of 2-S-substituted derivatives. In a series of 2-substituted derivatives, it has been recognized that flavor enhancing activities are influenced by Hammett σm and electrophilic superdelocalizability at the α-position of substituents (S2a), whereas thresholds are correlated with S2a and π. In a series of 2-S-substituted derivatives, it has been shown that π, D, and electrophilic superdelocalizability of the methylene protons in the -CH2S-group of substituents play important roles in the structure-activity relationships.