Linoleic acid and monomethyldecenylsuccinic acid were tested as uncoupling agents for energy linked functions of corn mitochondria. 2,4-dinitrophenol was used as a standard for comparison. Both compounds uncoupled oxidative phosphorylation, released oligomycin-blocked respiration, and accelerated adenosine triphosphatase. Linoleic acid uncoupled calcium-activated phosphate accumulation and the increase in light scattering that accompanies the accumulation. Unlike dinitrophenol, linoleic acid at 0.1 mm had a destructive effect on membrane semipermeability. Kinetic studies indicated that dinitrophenol and linoleic acid compete with phosphate for active sites in oxidative phosphorylation.Some linoleic acid is taken up by respiring mitochondria and a major share of the uptake is incorporated into phospholipids. Calcium ion and oligomycin promote the uptake, but coenzyme A does not. It is deduced that fatty acid probably attacks the non-phosphorylated intermediate, I approximately X, producing X approximately acyl. Uncoupling results from breakdown of X approximately acyl, but sufficient X approximately acyl is maintained to serve as a source of activated fatty acid.