Abstract In this memoir the following reactions have been studied:— 1. The Action of Hydrochloric and Acids on Opianic Acid. When strong hydrochloric or hydriodic acid acts at 100° for some time on opianic acid, iodide or chloride of methyl is evolved and a new acid formed, C10H10O5+ HCl= C9H8O5 + CH3Cl. We propose to call this acid methyl nor-opianic acid, as it stands intermediate between opianic acid and the normal opianic acid:— Normal opianic acid ....... C8H6O5 Methyl nor-opianic acid ..... C9H8O5 Opianic acid or dimethyl nor-opianic acid. C10H10O5 The new acid is soluble in cold water, but much more so in hot, from which it crystallizes on cooling with 2½ molecules of water. Like hypogallic acid it strikes a dark blue with sesquichloride of iron; but on addition of ammonia in excess, alight-red solution is produced, differing, therefore, from the hypogallic-aeid blue, with which ammonia becomes bloodred. From the analysis of the silver-salt it appears that methyl noropianic acid is monobasic.