It has been recognized since the 1950's that radioisotopic disequilibrium between naturally occurring parent-daughter isotope pairs in seawater can provide information on the rate of processes segregating them. In the surface ocean, the U-238: Th-234 parent-daughter pair tends to display measurable disequilibrium that is attributable to the effects of scavenging on particle reactive thorium. A range of approaches has been applied to deriving quantitative information on particle scavenging and sinking rates from U-238:Th-234 measurements. Here, we review the mathematical underpinnings of a very basic conceptual model of the removal process in order to correct a misunderstanding that has arisen in recent literature. ¿ 2001 American Geophysical Union