Mitochondria are critical sources of hydrogen peroxide (HO), an important secondary messenger in mammalian cells. Recent work has shown that O/HO emission from individual sites of production in mitochondria is regulated by protein S-glutathionylation. Here, we conducted the first examination of O/HO release rates from cardiac and liver mitochondria isolated from mice deficient for glutaredoxin-2 (GRX2), a matrix-associated thiol oxidoreductase that facilitates the S-glutathionylation and deglutathionylation of proteins. Liver mitochondria isolated from mice heterozygous (GRX2+/-) and homozygous (GRX2-/-) for glutaredoxin-2 displayed a significant decrease in O/HO release when oxidizing pyruvate or 2-oxoglutarate. The genetic deletion of the Grx2 gene was associated with increased protein expression of pyruvate dehydrogenase (PDH) but not 2-oxoglutarate dehydrogenase (OGDH). By contrast, O/HO production was augmented in cardiac mitochondria from GRX2+/- and GRX2-/- mice metabolizing pyruvate or 2-oxoglutarate which was associated with decreased PDH and OGDH protein levels. ROS production was augmented in liver and cardiac mitochondria metabolizing succinate. Inhibitor studies revealed that OGDH and Complex III served as high capacity ROS release sites in liver mitochondria. By contrast, Complex I and Complex III were found to be the chief O/HO emitters in cardiac mitochondria. These findings identify an essential role for GRX2 in regulating O/HO release from mitochondria in liver and cardiac tissue. Our results demonstrate that the GRX2-mediated regulation of O/HO release through the S-glutathionylation of mitochondrial proteins may play an integral role in controlling cellular ROS signaling.