The residence times of colloidal and particulate matter have been estimated often by using naturally occurring 234Th-238U disequilibria in the surface ocean. However, we show that the conventional 234Th multiple-box models underestimate the actual residence time. Using a one-box 234Th model approach, we calculate about a factor of two to four longer residence times for both colloidal and particulate thorium in the open ocean. This approach also yields very similar residence times between particulate thorium (box model based) and particulate organic carbon (sediment trap based) even with the same data sets previously used for showing a large discrepancy. Our results suggest significant corrections in the application of 234Th based residence times of the colloids and particulate matter to gauge the removal rates of reactive trace elements and organic carbon in the surface ocean. ¿ 1999 American Geophysical Union