Alkyl perxanthates (ROCSSO(-)) can be determined in solutions from flotation plants or other systems by direct ultraviolet spectrophotometry or after extraction into a solvent. Direct determination is preferred for visually clear solutions. In alkaline solution (pH > 8) at 348 nm perxanthates have a molar absorptivity of (1.042 +/- 0.013) x 10(4) l.mole(-1).cm(-1) and Beer's law holds up to 25 mg/l. The detection limit with a 1-cm cell is 0.2 mg/l. Interferences are few; nickelocyanide (>30 mg/l.) interferes slightly. After acidification to pH < 2, with an aqueous:organic phase-volume ratio of 1:1, more than 99% of the perxanthate is extracted into chloroform. In chloroform perxanthic acids have an apparent molar absorptivity of (5.47 +/- 0.09) x 10(3)l.mole(-1).cm(-1) at 302 nm. The absorbance of the chloroform extract at 302 nm is proportional to perxanthate concentration in the original aqueous phase up to 25 mg/l. The detection limit with a 1:1 phase-volume ratio and a 1-cm cell is 0.2 mg/l. Interferences include mercaptobenzothiazole (>1 mg/l.), sodium sulphite (>25 mg/l.) and cuprocyanide; xanthate interferes if it is not decomposed before extraction.