Abstract: Amethod for determining contents of Fe and Mo in the model compound of active center of nitrogenase by air-acetylene flame atomic absorption spectrophotometry was established. In the method the second strongest line of Fe and the strongest line of Mo were used so that the contents of Fe and Mo can be determined sequentially in the same sample solution. The sample is digested with concentrated nitrate acid, and the pHof sample solution is controlled in the range of l-2. There is no interference in the determination of Fe. The determination pf Mo is interfered by Fe, but this can be eliminated by adding,0,1 N NH₄Cl. The precision of the method is 1.4% for Fe and 1.0% for Mo, and the recovery is 100±2% for Fe and 100±4% for Mo.