It is demonstrated that the shape of MAgnetic field effect on Reaction Yield (MARY) spectra depends substantially on the relative signs of the hyperfine coupling constants (HFC) of the constituents of the transient radical pair. The effect is exemplified in terms of the pyrene/benzene-1,4-dicarbonitrile system, the spin dynamics of which are calculated with rigorous account of all HFCs and the peculiarities caused by a degenerate electron exchange reaction. It is shown that MARY spectra calculated using the same sign for all HFCs and those obtained using the signs as predicted from ab initio calculations differ significantly with respect to the amplitude of the low magnetic field feature. (c) 2005 Elsevier B.V. All rights reserved.