We formulate an ab initio method of quasirelativistic calculations on angular momentum and magnetic transition matrix elements between adiabatic electronic states of molecules. Our approach is based on the construction of a state-selective effective Hamiltonian and transition density matrices by means of the multireference many-body perturbation theory. Pilot applications to the evaluation of B0(u)(+) - B"1(u) predissociation matrix elements in I-2 and interactions in the B0(u)(+) similar to B1(u) complex of Te-2 are reported. (C) 2002 Published by Elsevier Science B.V.