Conjugative interactions of the carbon-carbon double bond are fundamental in organic chemistry. In this work, equilibria are established among conjugated and unconjugated isomers of two beta-substituted styrenes, 1-phenylbut-1-ene and 1-phenyl-3-methylbut-1-ene, and one alpha,beta-disubstituted styrene, 2-phenyl-5-methylhex-2-ene, over a range of temperatures (the van't Hoff method) in hexamethylphosphoric triamide and potassium tert-butoxide. From the trans styrenes of the first two sets, an enthalpy of conjugative interaction of phenyl vis-a-vis alkyl (Delta Delta H-ConjInter/Alk) = -2.5 +/- 0.2 kcal/mol [-5.1 kcal/mol defined as phenyl vis-a-vis hydrogen (Delta Delta H-ConjInter/H)] is observed, while the cis styrenes reveal an attenuated Delta Delta H-ConjInter/Alk of -1.1 kcal/mol (Delta Delta H-ConjInter/H = -2.7 kcal/mol). The alpha-methyl group in the third set also leads to a reduced conjugative interaction. Entropy plays an important role in determining positions of equilibrium. Free energies of conjugation are reported for several sterically hindered o-methyl-substituted styrenes.