The application of colourimetric cavity ringdown spectroscopy to the detection of trace compounds in water has been investigated using nitrite and iron(II) as test analytes. Samples were contained within one of three commercially available flow cells ranging in optical path length from 0.1 mm to 2.0 mm, and positioned within a two-mirror ringdown cavity. A measurement of the decay rate of the intensity of an optical pulse introduced into the cavity allows an ultrasensitive determination of optical absorption by the sample. A calibration using the known absorption coefficient of potassium permanganate at 532 nm was first carried out in order to determine the detection sensitivity in terms of minimum detectable absorption per unit path length when using each flow cell. The detection of nitrite and iron was then carried out by using well-known colour reactions, namely the Griess reaction for nitrite and the bathophenanthroline method for iron(II), to convert the analytes into strongly absorbing derivatives, which were quantified by a cavity ringdown measurement. In this first application of colourimetric cavity ringdown spectroscopy to the liquid phase, detection limits of 1.9 nM for nitrite and 3.8 nM for Fe(II) were demonstrated in a flow cell of path length 1.0 mm. The volume of sample analysed is only 196 nL, so that detection limits of this order correspond to the detection of less than 1 billion molecules. The detection method is therefore suitable for integration into a microfluidic sensing platform.