Accurate determination of the POC/234Th ratio on sinking particles is essential for the application of 234Th as a proxy for the export of particulate organic carbon (POC) from the upper ocean. Previous studies have shown that POC/234Th ratios may vary by as much as two orders of magnitude, but the mechanism for this variability remains poorly understood. In this study, POC/234Th and POC/228Th ratios in three particle size classes were examined for samples collected from the upper 500 m at a station in the South China Sea. Our results indicate that changes in the POC/234Th ratio with particle size at a given depth are mostly driven by simple decay of 234Th. In fact, various trends in the POC/234Th ratio versus particle size are explained by a combination of 234Th decay and the preferential remineralization of POC relative to Th during particle aggregation. We also observed a trend of decreasing 234Th/228Th ratios with particle size. This trend is consistent with results expected from a Brownian-pumping model.