The photo-oxidations, sensitised by proflavine, of furfuryl alcohol, N-allyl thiourea and histidine as a function of pH and substrate concentration have been investigated. The rate of oxidation of the neutral compound furfuryl alcohol is constant over the pH range of 9–3 and decreases below pH 3. This decrease corresponds to a change in the fluorescence spectrum of proflavine (fig. 4). A similar decrease in the oxidation rate of N-allyl thiourea at low pH is preceded by an unexpected increase in rate from pH 5 to pH 2 (fig. 5). The oxidation rate of histidine is constant from pH 9 to pH 7, drops below pH 7 and is virtually zero at pH 4 (fig. 6). This can be interpreted quantitatively to indicate that free imidazole groups and not positively charged imidazole groups are oxidized, provided the actual dependence of the oxidation rate on substrate concentration is taken into account (fig. 8). This dependence of the oxidation rate on the concentration of the three substrates can be described in terms of simple reaction kinetics (equation 8).