In the scheme of analysis of plant extracts described in a former paper (Davis and Daish, J. Agric. Sci., 1913, 5, 437), before it is possible to calculate the proportion of dextrose and laevulose an allowance must be made for the pentoses present; it therefore became necessary to ascertain the exact value of the cupric reducing power of these sugars under the standard conditions adopted—namely those defined by Brown, Morris and Millar. Value s of the cupric reducing power of xylose have been given already by Stone, Weiser and Zeitschek, and of arabinose by the latter workers and by Ost; but as they were obtained under conditions different from those specified by Brown, Morris and Millar, they are unsuitable for the present purpose, and it became necessary to make a fresh series of determinations. Xylose and arabinose are the only pentoses which are at all readily obtainable and in the present paper attention is limited to these.