Heterodimeric electron transfer flavoprotein that accepts electrons from several mitochondrial dehydrogenases, including acyl-CoA dehydrogenases, glutaryl-CoA and sarcosine dehydrogenase. It transfers the electrons to the main mitochondrial respiratory chain via ETF-ubiquinone oxidoreductase (ETF dehydrogenase). Required for normal mitochondrial fatty acid oxidation and normal amino acid metabolism. Binds 1 FAD per dimer. Heterodimer composed of ETFA and ETFB. Identified in a complex that contains ETFA, ETFB and ETFRF1. Interaction with ETFRF1 promotes dissociation of the bound FAD and loss of electron transfer activity (By similarity). Interacts with TASOR (By similarity). Domain I shares an identical polypeptide fold with the beta subunit ETFB though there is no sequence similarity. Belongs to the ETF alpha-subunit/FixB family.