Inter-relationship between the Hammett equation and the linear enthalpy-entropy effect has been discussed by deriving a new set of equations; ${Delta}{Delata}H^{ eq}=a{sigma}+b{Delta}{Delta}S^{ eq}$ and ${Delta}{Delta}F^{ eq}=a{sigma}+(b-T){Delta}{Delta}S^{ eq}$ where a = -1.36p. Theoretical analysis show that the Hammett, Leffler and Brown equations are special limited forms of these general equations. A necessary and sufficient test of substituent effect can thus be provided by the plot of $({Delta}{Delta}H^{ eq}-a{sigma)$ versus ${Delta}{Delta}S^{ eq}$<TEX>.