Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVDAT) binds efficiently pyrimidine derivatives from water solutions by hydrogen bonding. The order in the adsorbing activity (thymine, uridine, uracil, thymidine >> cytidine, cytosine > pyrimidine = 0) coincides with the number of hydrogen bonds formed between the guests and the 4,6-diamino-1,3,5-triazine residues in PVDAT. Purines and their derivatives are also adsorbed from their aqueous solutions by PVDAT. Hydrogen bonding is confirmed by the shift of the carbonyl stretching band of uracil toward a smaller wavenumber upon adduct formation with PVDAT. The association constant of the PVDAT-thymine adduct in water (150 M-1) is comparable with the corresponding value for the 4,6-diaminotriazine-thymine adduct in chloroform (90 M-1). Neither a monomer model of PVDAT nor its dimer model shows guest-binding activity in water. The hydrogen bonding of PVDAT is strengthened by benzene moieties attached to the diaminotriazine residues. The polymer microenvironment is essential for this molecular recognition through hydrogen bonding in water.