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. Required for normal mitochondrial fatty acid oxidation and normal amino acid metabolism. ETFB binds an AMP molecule that probably has a purely structural role. Heterodimer composed of ETFA and ETFB. Identified in a complex that contains ETFA, ETFB and ETFRF1. Interacts with ACADM. The recognition loop recognizes a hydrophobic patch at the surface of interacting dehydrogenases and acts as a static anchor at the interface. Methylated (PubMed:25023281). Trimethylation at Lys-200 and Lys-203 may negatively regulate the activity in electron transfer from acyl-CoA dehydrogenases. Belongs to the ETF beta-subunit/FixA family.