The anion exchange resin Bio-Rad AG1-X8 can be used to separate Mo from matrix elements in black shale and other geological samples. Most matrix elements, such as Fe, Ca, Na, Mn, Mg, Sr, Al, Cu and K, were eluted firstly by 1 mol/L HF+0.5 mol/L HCl; then Ti and Zr were eluted by 4 mol/L HCl; and Mo was eluted lastly by 3 mol/L HNO₃. Sr, which leads Mo isotope fractionation when Mo isotopic composition is analyzed by MC-ICP-MS, can be separated completely from Mo. During the eluting, most Mo was eluted within the first 5 mL HNO₃; the heavier Mo isotopes were collected first, then the lighter ones, so the eluant must be collected completely to avoid Mo isotope fractionation. When the amount of Mo in sample is large enough (≥50 μg), "column matrix effects" can be neglected and the Mo isotopic composition is unchanged after ion exchange separation, so the separation method is suitable for both sample-standard bracketing and double spiking techniques to calibrate the instrumental mass bias; nevertheless, due to "column matrix effects", this method is suitable for double spiking only when the amount of Mo is small (<50 μg).