Controlled laboratory experiments were carried out using pure sorbents to examine the interactions of 234Th (or 233Pa) with particle surfaces and whether there is a fractionation between 234Th and 233Pa by particulate chemical composition. Our results show that the adsorption of 233Pa and 234Th on particle surfaces is highly selective and compound specific, resulting in a fractionation of 233Pa/234Th ratio, especially on SiO2 (Kd ratio, Kd-233Pa/Kd-234Th, = 10) and CaCO3 (Kd ratio, Kd-233Pa/Kd-234Th, = 0.01) surfaces. These new results agree well with previously published field observations that showed an increase of 231Pa/230Th ratio (or Kd of 231Pa) with increasing opal abundance and decreasing carbonate content in oceanic particles. In addition, they highlight the complications of using 231Pa/230Th ratio as a proxy and have implications for both oceanography and trace element geochemistry although more studies are needed to further examine the influence of natural organic matter on the fractionation of Th/Pa ratios.