It is shown that the temperature-dependence of the Hammett equation is, in contrast to tradition, both physically and experimentally better described by means of temperature-dependent s and temperature- independent ? (termed ?o). The relationship between ?o and the customary (temperature dependent) ? is ?T = ?o(1/T-1/Tbiso)/(1/T-1/Tbiso) where Tbiso , is the isoequilibrium temperature of the benzoic acid ionization, for which the present analysis suggests a value of -255 K, and T is 298 K. In these terms, the temperature variation of the Hammett equation can be evaluated by supplying merely E(u)a (the activation energy for the reaction of the unsubstituted reactant) and ?o, in that the s value for the isokinetic substituent , i.e., the abscissa of the common point of intersection in the Hammett plot, is siso = (1/T-1/Tbiso)E(u)a/(2.303R?o) = E(u)a/(2630po) Further, ?o I related to energies ?o = E(u)a/(?H°u-?H°s(iso)) where ?H°u and ?H°s(iso) are the ionization enthalpies of the parent benzoic acid and that bearing the isokinetic substituent , respectively. Analogous equations apply to thermodynamic reaction series when substituting E(u)a for ?H°u(series). Along these lines the interpretation of the customary Hammett plot is advanced.