Biochemical changes associated with mushroom browning in Agaricus bisporus and Pleurotus florida were studied. With increasing storage temperature from 0 to 25 • C there was an increase in phenol oxidase activity up to 15 • C followed by a decrease at 25 • C in both mushrooms. Loss of water content in fresh mushrooms had a direct relationship with phenol oxidase activity, which could be correlated with the visual degree of mushroom browning. A bisporus displayed higher phenol oxidase activity, about two to three times that of P florida. The phenol oxidase activity in both mushroom varieties was studied on a range of phenolic compounds with diverse functional groups. A bisporus exhibited intense reactions to tyrosine and catechol, while P florida did so to guaiacol and catechol. The two mushrooms differed in the degree of their oxidation reactions relative to the functional groups. In A bisporus the skin had more phenolics than the flesh; P florida contained fewer phenolics, while the stalks of both mushrooms had low phenolics contents. FTIR spectral studies of intact mushrooms during browning showed a characteristic decrease in phenolic hydroxyls; first-derivative spectra were used to assess relative peak intensities. Of the chemicals tested for their effect on phenol oxidase activity, 0.1 M sodium carbonate favoured the immediate development of an orange chromogen in P florida, while mild alkaline solutions favoured the enzyme activity, and acidic solutions at the 0.1 M level completely inhibited the browning reaction in both mushrooms.