1. 1. Studies were made of potato oxidase action by determining the depth of color formed by the reaction of catechol and p-cresol with potato maceration juice under standard conditions. Reproducibility was somewhat greater with p-cresol than with catechol. 2. 2. The p-cresol oxidation proceeds steadily at a diminishing rate. Catechol color formation reaches its maximum at about 2 minutes, presumably due to enzyme inhibition by the catechol and thereafter shows little variation. 3. 3. Maximum color formation with catechol occurred at the natural p H of the juice. Maximum color formation with p-cresol was achieved in slightly alkaline solution which probably represents not the optimum p H of the reaction but the point at which enzymatic inactivation overcomes a greater chemical color formation in alkaline solution. 4. 4. With both substrates, the reaction is diminished by blanching in boiling water and completely extinguished in a sixty-second blanching period. 5. 5. The temperature curves of the catechol and cresol reactions are essentially similar except in the low temperature range. 6. 6. The coefficients of correlation of the catechol and cresol assays with the previously described direct colorimetric method and the o-phenylenediamine colorimetric and fluorometric methods were determined in 50 separate batches of potato dice. Almost complete correlation was found between cresol and catechol assays and between the o-phenylenediamine colorimetric and fluorometric methods. High positive correlations were found between the direct colorimetric method and the two o-phenylenediamine methods. Substantial positive correlations were found between the cresol and catechol assays and the other three methods. 7. 7. The o-phenylenediamine fluorescence method showed a high correlation with the dihydroxyphenylalanine content, and a slight positive correlation with the tyrosine content of the fresh potato.