The rate-limiting step of orthophosphate absorption by barley roots has been shown by kinetic studies (5) to be coupled to two of the sites of oxidative phosphorylation associated with isolated mitochondria. One of the reactions is coupled to reduced cytochrome b and the other to reduced diphosphopyridine nucleotide (DPNH). The present paper describes the identification of some of the products of the rate-limiting absorption step. The significance of the distribution of absorbed phosphate among these products and the rate of appearance in the products is discussed.