A theoretical method for describing magnetic-exchange-interact ions in covalent molecular systems is established by introducing covalent effect into Girerd-Journaux-Kahn's (GJK) magnetic formula and adopting our Double-Slater-Function (DSF) calculation method. The calculations show that the exchange coupling parameter J depends sensitively on the covalent factor N: the stronger the covalent effect the stronger the antiferromagnetic exchange interaction; the direct- and kinetic-exchange interactions are both important. The calculation for Mn-III-O-Mn-III spin-clusters in Mn2O(5-NO(2)saldien)(2) reveals that about 30% of the antiferromagnetic contribution comes from covalent effect, and that the theoretical values J = -111.02 to - 138.85 cm(-1) agree well with the experimental findings (J((expt.)) = -120 cm(-1)). (C) 2002 Elsevier Science B.V. All rights reserved.