In the buccal test there is a linear relation between percentage absorption and alkyl chain length for a series ofp‐n‐alkyl phenylacetic acids. Branching and cyclizing the alkyl chain reduces the percentage absorption. With thep‐halogeno‐acids, absorption increases as the atomic weight of the halogen atom increases; a chlorine atom is approximately equivalent to a methyl group in its effect on absorption. An oxygen atom between an alkyl group and ring is equivalent to reducing the chain length by one methylene group. The absorptions of α‐methyl‐substituted acids are greater than those of the unsubst