The document describes the construction and operation of an automated instrument package designed to measure gaseous and aqueous phase hydrogen peroxide. The chemical determination relies on the peroxidase-mediated conversion of p-hydroxyphenylacetic acid to 6,6'-dihydroxy-3,3'-biphenyldiacetic acid by H2O2 and subsequent base-induced ionization of all carboxylic and phenolic protons to form a fluorescent product. Organic hydroperoxides (hereafter referred to as organic peroxides) react in a similar fashion. The detection limit for gas phase H2O2 is 3 x 10 to the minus 11th power v/v (sampling rate 2 L/min) with a lag time of 1 min and 0-90 percent rise time of 0.8 min.