The partitioning behaviour for the trace elements Rb, Sr, Ba, Th, U, Zr, Hf, Nb, Ta, La, Ce, Sm, Ho, Lu and Y between immiscible Fe-rich, Si-poor and Fe-poor, Si-rich silicate liquids in the system KzO-AhOrSiOz-FeOFez03-P20s at 1265[degree]C, 1 atm was determined as a function of oxygen fugacity in the range OflOg/02 from -8.7 to -5.7. The miscibility gap for all major elements is enlarged and the partition coefficients for Sr, Ba and U increase and Rb decreases with increasing /02' The partition coefficients for the remaining trace elements stay nearly constant. Thus it is predicted that the partitioning of Sr, Ba, U and Rb between crystals and melt will show increasing dependence on melt composition as /02 is increased, but the compositional dependence of the crystal/melt partition coefficients for the other elements will remain unchanged.