The determination of trace level impurities in nuclear materials was performed by inductively coupled plasma mass spectrometry (ICP-MS). Ebr this purpose extensive modifications of a commercial ICP-MS instrument were carried out and the operating conditions optimized, The effect of Zr, Th, U and Pu at concentrations of up to 1000 mu g g(-1) on the signal of various isotopes was studied. A strong signal suppression effect at concentrations higher than 100 mu g g(-1) was found for U and Po. The effect of Zr and Th was found to be lower, Special effort was made to improve the detection limit for B; the detection limit for B in PuO2 could be lowered to less than 1 mu g g(-1).