The effect of tetradecyl-dimethyl-benzyl-ammonium chloride (zephiramine=Z+Cl-) on uranium with 4-(2-thiazolylazo) resorcinol (TAR) was studied. In the presence of zephiramine, a uranium-TAR complex was stabilized and solublized in aqueous media at a pH around 7. The addition of zephiramine made it possible to use phosphate buffers which have large buffer capacity in the optimum range for the determination of uranium. Almost all interfering ions can be masked with an EDTA solution. As a result, a sensitive and simple method for the determination of uranium was developed.