University of Craiova, A. I. Cuza 13, 1100 Craiova, Romania E-mail : protaru@central.ucv.ro Fax : 40-251-415077 Manuscript received 21 January 2002, revised 14 November 2002, accepted 20 November 2002 Presence of inert gas has no influence on equation that one uses to calculate the isoconversion curves with Kx =constant. The curve with Kx = 1 (or ΔG\(^P_T\)= 0) is called 'curve of normal null affinity'. In case of reactions with volume change, that take place in isothermal-isobaric conditions, the influence of inert gas on conversion ξ is maximum, if the reactions take place in conditions of temperature and pressure situated nearly the curve of normal null affinity. For mentioned reactions, the change of conversion Δξ increases with the relative change of volume Δn/ng.