1. The effects of two, cyclic, non-substituted monoterpenes (betapinene and limonene) were studied on isolated mitochondria. Oxidative phosphorylation was inhibited depending on monoterpene concentration and on monoterpene association (droplet size) in aqueous suspension. 2. In the absence of hydration, both monoterpenes, dissolved in either dimethylformamide (DMF) or dimethylsulphoxide (DMSO), inhibited oxidative phosphorylation through depletion of the transmembrane potential and inhibition of oxygen consumption. Hydration of the monoterpene solutions before addition to mitochondria resulted in milder inhibitory effects. The decrease in inhibition was more pronounced with DMSO than with DMF. 3. The possibility that the biochemical properties of a compound in suspension may depend on molecular association should be considered whenever studies of a hydrophobic compound in an aqueous environment are conducted.