Flash photolysis of 1H-benzobiazole (1) has shown that N-N bond fission occurs within a few picoseconds of excitation (248 nm) giving 6-diazo-2,4-cyclohexadien-1-imine (2), lambda(max) = 430 nm. Fluorescence and intersystem crossing do not compete measurably, and photodenitrogenation is a minor side reaction (<1%). The rate constant for recyclization in the ground state, 2 --> 1, is on the order of 2 x 10(9) s(-1) in aprotic solvents, and 3.2 x 10(7) s(-1) in aqueous solution. DFT calculations gave Delta(2-->1)G(double dagger) = 3.7 and Delta(2-->1)G(0) = -28.0 kcal mol(-1) for the activation and reaction free energies, respectively. Imine 2 is a moderately strong base, pK(a) = 12.7, that is protonated by water yielding 2-aminophenyl diazonium ion (2(+)), lambda(max) = 405 nm. The rate constant for cyclization of the cation, 2(+) --> 1 + H+, is k = 3 s(-1). The reversible photoreaction of 1 may be used to induce an upward pH jump in unbuffered water, or as a kinetic pH-indicator for the range of 6-12 in buffered solutions. An erroneous Value for the acidity constant of protonated 1 is corrected to pK(a)(1(+)) = 0.42 +/- 0.02 (I = 1.0 M). Several derivatives of 1 were also investigated.