Electron impact excitation cross sections of dipole-allowed transitions in diatomic molecules have been evaluated using the similarity function method allowing one to express cross sections through the oscillator strength of the transition and some universal function of the reduced incident electron energy. The necessary characteristics of the molecular terms involved and transition dipoles were determined by ab initio electronic structure calculations using the multireference configuration interaction technique or relativistic multipartitioning many-body perturbation theory. The results of specific calculations pet-formed for CO and NO are in a reasonable agreement with the available experimental data and the results of comprehensive quantum-mechanical calculations. (c) 2007 Elsevier B.V. All rights reserved.