Equilibrium constants (K-de) are reported for the dehydration of hydrates of benzene, naphthalene, phenanthrene, and anthracene. Free energies of formation of the hydrates (DeltaGdegrees(f)(aq)) are derived by combining free energies of formation of the parent (dihydroaromatic) hydrocarbon with estimates of the increment in free energy (DeltaG(OH)) accompanying replacement of a hydrogen atom of the hydrocarbon by a hydroxyl group. Combining these in turn with free energies of formation of H2O and of the aromatic hydrocarbon products furnishes the desired equilibrium constants. The method depends on the availability of thermodynamic data (i) for the hydrocarbons from which the hydrates are derived by hydroxyl substitution and (ii) for a sufficient range of alcohols to assess the structural dependence of DeltaG(OH). The data comprise chiefly heats of formation and standard entropies in the gas phase and free energies of transfer from the gas phase to aqueous solution (the latter being derived from vapor pressures and solubilities). They also include experimental measurements of equilibrium constants for dehydration of alcohols, especially cyclic, allylic, and benzylic alcohols. In general DeltaG(OH) depends on whether the alcohol is (a) primary, secondary, or tertiary; (b) allylic or benzylic; and (c) open chain or cyclic. Differences in geminal interactions of the hydroxyl group of the alcohol with alpha-alkyl and vinyl or phenyl groups account for variations in DeltaG(OH) of 5 kcal mol(-1). Weaker variations which arise from beta-vinyl/OH or beta-phenyl/OH interactions present in the aromatic hydrates but not in experimentally studied analogues are estimated as 1.0 kcal mol-1. Equilibrium constants for dehydration may be expressed as their negative logs (pK(de)). Reactions yielding the following aliphatic, aromatic, and antiaromatic unsaturated products then have pK(de) values: +4.8, ethene; +15.0, ethyne; +22.1, cyclopropene; +28.4 cyclobutadiene; -22.2, benzene; -14.6, naphthalene; -9.2, phenanthrene; -7.4, anthracene. Large positive values are associated with formation of strained or antiaromatic double bonds and large negative values with aromatic double bonds. Trends in pK(de) parallel those of heats of hydrogenation. The results illustrate the usefulness of a substituent treatment for extending the range of currently available free energies of formation. In addition to hydroxyl substituent effects, DeltaG(OH), values of DeltaC(pi) for substitution of a pi-bond in a hydrocarbon are reported.