As a means of avoiding side reactions which were encountered in earlier studies of hydrogen-deuterium exchange of phenol ethers with ethanol-sulphuric acid mixtures, a new procedure has been devised for the study of exchange reactions in acetic acid. The analysis for deuterium in the carboxyl group of acetic acid is performed by adding methyl magnesium iodide solution to the solution of phenol ether in acetic acid, combustion of the methane so produced, and finally, by determining the density of the water formed in the combustion. As compared with the former procedure, which required fractionation of the reaction mixture to recover the pure constituents, advantages in convenience and accuracy are claimed. Results are reported for the following ethers, reacting at 90 °C. in the presence of trace quantities of sulphuric acid: anisole, o-methylanisole, m-methylanisole, 3,5-dimethylanisole, 2-methylcoumarane, veratrole, di-isobutyl catechol ether, and the cyclic mono- (di-, tri-, and tetra) methylene ethers of catechol. The observed variations in rates are interpreted in part according to whether the conditions imposed by structural alterations favor or constrain the formation of an intermediate cation for which a quinoidal configuration of bonds is required. The role of steric factors is most strikingly demonstrated in the series of cyclic ethers of catechol, constraint being evident at a critical ring size (7-membered ring) and largely absent for either smaller or larger rings.