Expressions for contributions to T-1 and T-2 from modulation of the exchange interaction (J) in spin-correlated radical pairs (SCRPs) are derived and incorporated into SCRP theory for simulation of direct detection EPR spectra of alkane chain biradicals. The T-2 term is consistent with that previously derived by Luckhurst to explain the temperature-dependent alternating line widths observed in EPR spectra of stable nitroxide biradicals. Alternating line-width patterns are observed for symmetric bis(alkyl) biradicals at high temperatures, where the rapid modulation of J is caused by conformational jumping. However, in unsymmetrical systems such as acyl-alkyl biradicals, J modulation manifests itself at low temperatures as an increased broadening of the spectral lines with increasing distance of the transition from the center of the spectrum, with no alternation. Several experimental examples are presented, simulated, and discussed. Contrary to a suggestion by Maeda et al., J modulation is shown to have only a minimal effect on T-1 in SCRP theory for the case of biradicals undergoing rapid conformational interconversion. A connection is proposed between the magnitude of the relaxation matrix elements and the biradical conformational distribution and chain dynamics.