Selective reaction or photodissociation of isocyanic acid (HNCO) molecules in well-characterized vibrational eigenstates is a means of controlling their chemistry. The key to the measurements is the characterization of their vibrational states by absorption spectroscopy and the determination of their reaction efficiency by action spectroscopy. Absorption spectroscopy and theoretical calculations on states in the region of three quanta of the N-H stretching excitation (3 nu (1)) of HNCO identify couplings between the bright N-H stretching state and states that have one or more quanta of N-C-O bending excitation. Bimolecular reaction action spectroscopy, which monitors the yield of NCO from the reaction of Cl atoms with HNCO, and photodissociation action spectroscopy, which monitors the production of (NH)-N-1 following S-1 <-- S-0 excitation in HNCO, measure the relative reaction and photodissociation cross sections for the mixed vibrational eigenstates having rotational quantum numbers K = 3 and J = 6 and 7, The measurements and analysis show that the perturbing zero-order state is substantially less reactive than the bright state but has a photodissociation cross section that is much larger than that of the bright state. Bending excitation of N-C-O strongly influences both the reaction and photodissociation, hindering bimolecular reaction but promoting photodissociation.