The products of the reactive deactivation of O(1D) by HCN are monitored using laser fluorescence spectroscopy. The dominant channel for reaction leads to formation of NH(a 1Δ)+CO(X 1Σ+); less than 10% of the reactive encounters yield OH(X 2π)+CN(X 2Σ+). The rotational distribution of NH(a 1Δ) is significantly colder than prior prediction probably as the result of a kinematic effect, while that of CN(X 2Σ+) is slightly cooler, but both are generally well described by a Boltzmann temperature. There appears to be an inversion of the v″=1/v″=0 population ratio in NH(a 1Δ), although the present data confirm that previously calculated Franck–Condon factors for the c 1π←a 1Δ transition do not accurately reflect the observed intensities. The dominant reactive pathway involves insertion of O(1D) into the C–N bond to form an oxazirine intermediate, with subsequent migration of the hydrogen from carbon to nitrogen leading to formation of the observed products. This interaction proceeds initially on the 1A′ ground state surface, but analysis of orbital symmetry conservation reveals that a nonadiabatic transition to the low-lying 1A″ surface is required prior to hydrogen migration.