Long-range electron transfer (ET) in mixed-metal hemoglobin hybrids [(MP), Fe (CN~) P](M= Mg, Zn) is measured as a function of temperature from ambient to 100 K. Triplet-state quenching of 3 (MP) is found to be inadequate to determine ET rate constants when the quenching rate is low, as it is in these hybrids at low temperature. Direct observation of the ET intermediate,[(MP)+, Fe2+(CN~) P], has allowed us to determine the temperature response of the ET rate constants kt and kb for the 3 (MP)- Fe3+(CN~) P and Fe2+(CN~) P(MP)+ electron-transfer steps, respectively. The ET process is not affected by the freezing of the cryosolvent, which may indicate that coupling of ET to low-frequency solvent modes may be minimal. Forboth M, but especially for M= Mg, kb is nearly temperature independent. Calculationof the electron coupling matrix element gives HAB== 10~ 3 cm" 1. Comparison with results for ruthenated myoglobin indicates that the Hb interface exhibits efficient electronic coupling even across the noncovalent interface between the subunits.