The preparation and characterization of three new donor-bridge-acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), beta = 0.39 angstrom(-1) and oligo(2,54hiophene), beta = 0.22 A(-1). This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (beta) for these bridges. The work provides a direct measurement of superexchange contributions to beta, with no contribution from incoherent hopping. The different beta values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D-B energy gap, A, rather than bridge-bridge electronic couplings, H-BB. This is supported by the fact that the HBB values extracted from the experimental data for oligo(para-phenylene) (H-BB = 11 400 cm(-1)) and oligo(2,5-thiophene) (12 300 cm-1) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern H-DA and beta, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.