A spectrophotometric method with thiomalic acid for the determination of molybdenum was investigated, and applied to the direct determination of molybdenum in high alloy steel. The molybdenum thiomalic acid complex is formed quantitatively in a sulfuric acid-sodium acetate solution medium of pH 3.0∼4.5. The absorption maximum of this complex is at a wavelength of 365 nm.Hydrochloric, nitric and perchloric acids do not interfere with the formation of a molybdenum thiomalic acid complex, but phosphoric, tartaric, citric and oxalic acids interfere. The presence of sulfuric acid increases slightly the absorbance for the determination of molybdenum, but its effect appears constant in the range from 1 to 5 mL as sulfuric acid (1+1).The presence of copper, iron (II) and tungsten increases the absorbance for the determination of molybdenum, and the presence of niobium, tantalum, titanium and zirconium decreases. However, the effect of the copper can be masked by using thiourea, the effect of the iron (II) appears constant in the range from 10 to 20 mg, and the effect of the tungsten can be eliminated by measuring the absorbance at the wavelength of 400 nm. The effect of the niobium, tantalum, titanium and zirconium can be masked by using sodium fluoride.The sample was fumed with sulfuric acid containing ammonium sulfate for dissolving completely the precipitated niobium, tantalum and tungsten after the sample was dissolved in aqua regia containing ammonium bifluoride, and the sample solution was prepared by adding sodium fluoride for preventing the hydrolysis of the niobium, tantalum and tungsten in the sample solution.As a result of the experiment, the author succeeded in establishing a method in which 0.5 to 20% of molybdenum in high alloy steel can precisely be measured without difficulty.The molybdenum contents in actual samples were measured by this method with satisfactory results.