The distribution of uranyl chloride between aqueous hydrochloric acid solutions and indiluted tri-n-butyl phosphate (TBP) was examined as a function of increasing acid concentration in the initial aqueous solution, the metal concentration being kept constant, and as a function of increasing uranyl chloride concentration for constant acid content of the initial aqueous solution. The pregnant organic layers involved were investigated by means of volume-swelling determinations, and by viscosity, density, conductivity and spectrophotometric measurements. The water content of the tri-butyl phosphate layer was also determined. It was found that the chemical mechanism governing the extraction of uranyl chloride is largely dependent upon the hydrochloric acid concentration in the initial aqueous solutions. For low acid concentrations the mutual salting-out effect of thr two solutes controls the mechanism; at medium acid concentrations (1–7 M HCl) the acid concentration has no marked influence on the extraction mechanism involved; at high acid concentrations it is the competition between two complex-solvates, namely the hydrochloric acid-monosolvate-trihydrate and the uranyl chloride-disolvate, which controls the extraction of uranyl chloride from aqueous solutions.