Abstract: This paper shows the effect of macro amount of molybdenum on the extraction of micro amount of niobium by different solvents from an aqueous phase containing nitric acid, nitrate and molybdenum in various concentrations. Nio-bium-95 and molybdenum-99 tracers are used for the determination of distribution coefficients of niobium and molybdenum. Results indicate that there is an obvious effect of molybdenum on the extraction of.niobium from 0.4-1.0 mol/1 HNO3 by TBP, HDBP and Aliquat-7402. The distribution coefficient of niobium increases by 4 to 254 times with the addition of inactive molybdenum into the extraction system.Based on the experimental data, equations are suggested for the extraction equilibrium of niobium and molybdenum. The results of slope analysis show that the extracted niobium species by HDBP is a mixture in the following form: 2Nb(OH)(NO3)3H2(DBP)3 + LNb(OH)(NO3)2H2(DBP)4.