A highly sensitive and rapid method has been developed for the extraction-spectrophotometric determination of ultra trace amounts of americium. Americium(III) is selectively extracted from 1–10M HNO3 medium with a mixture of (0.3M HDEHP+0.1M P2O5), both dissolved in xylene, and finally estimated in the organic phase itself absorptiometrically employing Arsenazo-III as the chromogenic reagent. A 60% dioxaneethanol mixture was used for optium colour development. Beer's law is obeyed in the concentration range 0.1–0.7 g Am cm–3 and as little as 0.11 g Am cm–3 could be determined with a precision better than ±2%. The molar absorptivity based on Am content is (3.599±0.049)·105 dm3·mol–1·cm–1 at 648 nm which is incidentally the highest value reported as yet for its determination. The optimum concentration range, evaluated by Ringbom's method is 0.1–0.6 ppm Am. Common contaminants such as Al3+, Co2+, Cr3+, Fe3+, Ni2+, Zr4+, F–, NO3–, and SO
in fairly large quantities and moderate amounts of Pu4+, Th4+ and UO
cause no interference in the final assay. Colour development is almost instantaneous and its intensity remains virtually constant for at least 48 hours.