AbstractThis paper contains data on the (pseudo) catalytic deacylation of a number of derivatives ofortho‐nitro‐acetanilide in which steric effects on mesomerism are to be expected. The rate of deacylation found for each compound is discussed in some detail on the basis of current electronic theory.It is shown that the occurrence of steric inhibition of mesomerism can sometimes be used to advantage in the separation of mixtures of isomeric nitroaryl‐acylamines.