A mathematical model formulated as ΔfHθm(g)=a+sum from i=1 to 5bipi(i≠2) was constructed for the relationshipbetween standard formation enthalpy ΔfHθm(g) and path index Pi of substituted benzenes derivatives. An empiric equation for the calculation of ΔfHθm(g) was worked out. The calculated values of standard formation enthalpy based on this model are excellently consistent with those from experimental for 55 organic compounds. The model is shown to be simple and of practical usefulness, particularly when required experimental data are unavailable. A mathematical model formulated as ΔfHθm (g) = a + sum from i = 1 to 5bipi (i ≠ 2) was constructed for the relationshipbetween standard formation enthalpy ΔfHθm (g) and path index Pi of substituted benzenes derivatives. An empiric equation for the The calculated values ​​of standard formation enthalpy based on this model are excellently consistent with those from experimental for 55 organic compounds. The model is shown as simple and of practical usefulness, particularly when required experimental data are unavailable.