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The photo-oxidation of purine nucleotides adenosine-5'-monophosphate (AMP) and guanosine-5'-monophosphate (GMP) by 3,3',4,4'-benzophenone tetracarboxylic acid (TCBP) has been investigated in aqueous solutions using nanosecond laser flash photolysis (LFP) and time-resolved chemically induced dynamic nuclear polarization (CIDNP). The pH dependences of quenching rate constants and of geminate polarization are measured within a wide range of pH values. As a result, the chemical reactivity of reacting species in different protonation states is determined. In acidic solution (pH < 4.9), the quenching rate constant is close to the diffusion-controlled limit: kq = 1.3 × 10(9) M(-1) s(-1) (GMP), and kq = 1.2 × 10(9) M(-1) s(-1) (AMP), whereas in neutral and basic solutions it is significantly lower: kq = 2.6 × 10(8) M(-1) s(-1) (GMP, 4.9 < pH < 9.4), kq = 3.5 × 10(7) M(-1) s(-1) (GMP, pH > 9.4), kq = 1.0 × 10(8) M(-1) s(-1) (AMP, pH > 6.5). Surprisingly, the strong influence of the protonation state of the phosphoric group on the oxidation of adenosine-5'-monophosphate is revealed: the deprotonation of the AMP phosphoric group (6.5) decreases the quenching rate constant from 5.0 × 10(8) M(-1) s(-1) (4.9 < pH < 6.5) to 1.0 × 10(8) M(-1) s(-1) (pH > 6.5).