The association constants of hen egg yolk riboflavin binding protein with 8-substituted riboflavins were measured using a fluorometric titration method in the range of 10-40 degrees C, and thermodynamic parameters were calculated using ordinary methods. The flavins used were riboflavins whose 8-methyl group was substituted with HRN, RR'N, RO type groups, or halogen atoms. From a plot of delta H degrees against delta Su (unitary entropy change), the flavins were classified into four groups, i.e., HRN, RR'N, RO, and halogen type substituents. In each group, the linear free energy relationship, delta H degrees = u . delta Su+v (u and v were constants specific for each group), was found to be similar to that for egg white riboflavin binding protein, though with different specific constants for each egg protein. The relation between delta Su and the bulkiness (molecular volume) of 8-substituents suggested burying of the 8-substituents in a cavity of the protein similar to that of the egg white protein, but of smaller volume. The behavior of the halogen group was similar to that of the other three groups, and could be explained in terms of the bulkiness of the substituents rather than by assuming electric repulsion between halogen substituents and the binding site, which is in contrast with the case of egg white protein. A linear relation between v value and the wavelength of the visible absorption peak of flavins wa also found.