Chemical Laboratories, Directorate of Geology & Mining, Raipur, (M. P.) Department of Chemistry, Government College of Science, Raipur, (M. P.) Manuscript received 17 January 1977, revised 18 May 1978, accepted 22 December 1978 A rapid spectrophotometric method for the determination of trace amounts of molybdenum has been developed using phenylhydrazine sulphinic acid. An aqueous solution of the reagent develops a red colour with molybdate ions in acetic acid media ranging from 10 to 70% by volume. The intense red colour produced has its maximum absorbance at 510 nm. Sandell's sensitivity of the colour reaction is 0.012 µg of molybdenum cm2. The system conforms to Beer's law over the concentration range of 0.6 to 6 µg per ml of molybdenum and has a molar absorptivity of 8.0 x 102 litre mole-1cm-1. Investigations have indicated that the reagent is a useful alternative for sulphurous acid in the determination of copper as copper(I)-thiocyanate.