The hydroxyl radical is formed through a variety of processes pertinent to natural and anthropogenic systems. Here we report development of a simple and sensitive trap-and-trigger chemiluminescence method based upon the hydroxylation of phthalhydrazide to 5-hydroxy-2,3-dihydro-1,4-phthalazinedione, which emits chemiluminescence when oxidized under alkaline conditions. Cu(III) is employed as an oxidant and is shown to be relatively insensitive to a range of interferences likely to be encountered. The method has been standardized by use of γ-radiolysis of water as a primary source of hydroxyl radical, with a convenient secondary calibration procedure developed that uses Fenton chemistry. Detection limits varied from 7.4 nM (at pH 3) and 6.2 nM (at pH 8.1) of accumulated hydroxyl radical production in a simple 10 mM NaCl matrix to around 30 nM in an artificial seawater medium, due to competition for hydroxyl between the phthalhydrazide probe and bromide. The method has been used to characterize the kinetics of the Fenton system employed for calibration and is shown to be consistent with published models of this process over time scales of several hours. The application of this method to a range of matrices and for photochemical studies is also described.