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, rye 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.