The use of cavity ring-down spectroscopy for kinetic measurements of photo-induced reactions in solution has been demonstrated for the first time. Second-order rate coefficients for reaction of NO 3 with terpenes in acetonitrile were determined: (2.93+/-0.48)x10 9 M - 1 s - 1 (α-pinene), (2.86+/-0.35)x10 9 M - 1 s - 1 (β-pinene), and (1.40+/-0.12)x10 9 M - 1 s - 1 (limonene). The rate coefficients were found to be lower than previous measurements in the gas-phase, which we attribute to the onset of a diffusion-controlled limit to the reaction rate in solution.