The first mechanistic study of a spin-forbidden proton-transfer reaction in aqueous solution is reported. Laser flash photolysis of alkaline trioxodinitrate (N2O32-, Angeli's anion) is used to generate a nitroxyl anion in its excited singlet state ((NO-)-N-1). Through rapid partitioning between protonation by water and electronic relaxation, (NO-)-N-1 produces (HNO)-H-1 (ground state, yield 96%) and (NO-)-N-3 (ground state, yield 4%), which comprise a unique conjugate acid-base couple with different ground-state multiplicities. Using the large difference between reactivities of (HNO)-H-1 and (NO-)-N-3 in the peroxynitrite-forming reaction with 3 02, the kinetics of spin-forbidden deprotonation reaction (HNO)-H-1 + OH- --> (NO-)-N-3 + H2O is investigated in H2O and D2O. Consistent with proton transfer, this reaction exhibits primary kinetic hydrogen isotope effect k(H)/k(D) = 3.1 at 298 K, which is found to be temperature-dependent. Arrhenius pre-exponential factors and activation energies of the second-order rate constant are found to be: log(A, M-1 s(-1)) = 10.0 +/- 0.2 and E-a = 30.0 +/- 1.1 kJ/mol for proton transfer and log(A, M-1 s(-1)) = 10.4 +/- 0.1 and E-a = 35.1 +/- 0.7 kJ/mol for deuteron transfer. Collectively, these data are interpreted to show that the nuclear reorganization requirements arising from the spin prohibition necessitate significant activation before spin change can take place, but the spin change itself must occur extremely rapidly. It is concluded that a synergy between the spin prohibition and the reaction energetics creates an intersystem barrier and is responsible for slowness of the spin-forbidden deprotonation of (HNO)-H-1 by OH-; the spin prohibition alone plays a minor role.