The vapour pressure isotherms determined for n-hexane mixtures of n-butylamine between 283.15 and 373.15 K and of t-butylamine (2-methyl-2-propanamine) between 283.15 and 343.15 K are represented by Wilson’s equations. Considering the relation to the pair interactions of the components in the binary mixtures, the Wilson coefficients indicate an association of the two butylamines which is slightly weaker than that of the lower primary aliphatic amines, but somewhat stronger or at least not weaker than than that of the secondary analogues methylethyl-and diethylamine. Particularly, however, t-butylamine proves to be distinctly more weakly associated than n-butylamine. The same behaviour is suggested by the calculated thermodynamic functions including the excess functions. In agreement with observations for the vaporization enthalpies and the heats of mixing at infinite dilution, and also with the conclusions for n-propyl-and iso-propylamine, the weaker association of tertiary butylamine is mainly attributed to the shielding of the associating group, i.e. to the steric hindrance of N-H ... N-bond formation due to branching of the alkyl group.