A theory of nonadiabatic donor (D)-acceptor (A) two-electron transfer (TET) mediated by a single regular bridge (B) is developed. The presence of different intermediate two-electron states connecting the reactant state D(--)BA with the product state DBA(--) results in complex multiexponential kinetics. The conditions are discussed at which a reduction to two-exponential as well as single-exponential kinetics becomes possible. For the latter case the rate K-TET is calculated, which describes the bridge-mediated reaction as an effective two-electron D-A transfer. In the limit of small populations of the intermediate TET states D(-)B(-)A, DB(--)A, D(-)BA(-), and DB(-)A(-), K-TET is obtained as a sum of the rates K-TET((step)) and K-TET((sup)). The first rate describes stepwise TET originated by transitions of a single electron. It starts at D(--)BA and reaches DBA(--) via the intermediate state D(-)BA(-). These transitions cover contributions from sequential as well as superexchange reactions all including reduced bridge states. In contrast, a specific two-electron superexchange mechanism from D(--)BA to DBA(--) defines K-TET((sup)). An analytic dependence of K-TET((step)) and K-TET((sup)) on the number of bridging units is presented and different regimes of D-A TET are studied. (C) 2004 American Institute of Physics.