Recent studies suggest that benthic foraminiferal Mg/Ca may be useful in reconstructing deep water temperature changes during the Quaternary and on longer timescales <Rosenthal et al., 1997; Lear et al., 2000; Martin et al., 2002>. Because changes in deep water temperature may be small, sample preparation could play an important role in reconstructing accurate temperatures from shell Mg/Ca. A simple examination of changes in measured shell chemistry of benthic foraminifera from two intervals subjected to increasingly rigorous cleaning reveals that fossil benthic foraminiferal samples cleaned by oxidation yield lower Mg/Ca values than samples only rinsed and weak acid leached. Reductive cleaning further lowered the shell Mg/Ca of one of the two benthic species examined. Lower Mg/Ca values following oxidation and reduction appear to reflect removal of contaminant Mg associated with remnant organic matter and adsorbed phases.