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