AbstractFollowing up earlier work, this paper discusses the influence of 2‐methyl‐6‐tert.butyl substitution and/or 2,6‐di‐tert.butyl substitution on certain properties of compounds such as nitrobenzene, aniline, N‐methylaniline, N,N‐dimethylaniline, acetanilide, 4‐nitro‐aniline, and 4‐nitro‐acetanilide.For the derivatives of nitrobenzene, N,N‐dimethylaniline, and acetanilide the electronic absorption spectra indicate a complete or almost complete elimination of the mesomeric interaction. In contrast with this it has to be concluded for all the primary amines investigated that the mesomerism is fully developed; in the relevant discussion the vectorial treatment of absorption Intensities has been of decisive importance. The secondary amines take up an intermediate position; in these cases, owing to the pyramidal configuration of the nitrogen atom, the angle of twist cannot be forced to values exceeding about 60°.The basic strength of 2,6‐di‐tert.butyl‐substituted primary amines is lower by a factor of about 108than might be expected on the basis of the electrical effects of the alkyl groups; the basic strength of 2‐methyl‐6‐tert.butyl‐substituted N,N‐dimethylanilines is lower by a factor of about 106than might be expected on the basis of the electrical effects of the alkyl groups and the steric inhibition of the mesomerism present in these compounds. These reductions of the basic strength have been ascribed to steric hindrance to solvation.The basic strength of 3,5‐di‐tert.butyl‐4‐nitroaniline confirms previously obtained information on the quantitative significance of the inductive and the mesomeric effect of the nitro group in 4‐nitroaniline as we