Electron microscopic studies show that the electron density of the mitochondria of the thermogenic appendix of Sauromatum guttatum inflorescences changes during development. Two to 3 days before heat-production, the mitochondria accumulate osmiophilic, electron-dense material between the inner and outer membranes. During heat-production and the release of volatiles compounds the osmiophilic material disappears from the inter membrane space. Deposits with the same electron density are also found in the endoplasmic reticulum. It is concluded that the mitochondria may also have the capacity to accumulate that material in the inter membrane space.