Publisher Summary This chapter discusses the spectrophotometric determination of thallium from silicate rock material, using brilliant green. It is necessary to separate thallium from interfering elements, such as antimony, tin, mercury, cadmium, chromium, and tungsten. The chapter describes the solvent extraction of thallic bromide into diethyl ether for the separation of thallium from the interfering elements. The reaction between the anion TlBr4- and the brilliant green cation gives an organic-soluble green color complex with maximum absorption at the wavelength of 627 nm. Higher sensitivities for thallium are obtained from the methods that are based upon the quenching of the fluorescence of cochineal red and uranyl sulfate, or on the fluorescence given by thallium with rhodamine B, or of the thallous chloro complex. In all the cases, a prior separation using ion-exchange, solvent extraction, or co-precipitation techniques is required. Atomic absorption spectroscopic (AAS) methods based on the aspiration of the aqueous acid solution of rock material are not of sufficient sensitivity for determining the thallium content of most silicate rocks.