The equilibrium of the positional and structural isomerization of branched monopentylbenzenes, pentyltoluenes, and pentyl-o-xylenes was studied. It was found that the 1,2-dimethylpropyl substituted derivatives prevail over the 1,1-dimethylpropyl substituted isomers in the equilibrium mixture of all of the examined groups of compounds. The thermodynamic characteristics of the structural isomerization of pentylbenzenes were calculated from the experimental data.