We demonstrate the application of cavity ring-down spectroscopy (CRDS) to the measurement of concentrations of IO radical and of reaction rate coefficients for the reaction systems, IO + IO and IO + NO, using the source reaction, O(P-3) + CSI. By monitoring IO radicals, we obtain the 295 K rate coefficients, k(O + CSI --> IO + CF3) = (5.8 +/- 1.5) x 10(-12) cm(3) molec(-1) s(-1); k(IO + IO) = (1.0 +/- 0.3) x 10(-10) cm(3) molec(-1) s(-1) and k(IO + NO) = (1.9 +/- 0.5) x 10(-12) cm(3) molec(-1) s(-1) at the pressures of 1250 Pa (9.4 Torr) and 4000 Pa (30.1 Torr). For the IO A (2)Pi(3/2) - X (2)Pi(3/2) (2, 0) bandhead at 445.04 nm we have determined an absorption cross-section, sigma = (7.3 +/- 0.7) x 10(-17) cm(2). Error limits indicate the confidence of two standard deviations and propagate the uncertainty in the absorption cross-section.