Although coccolith calcite is abundant in carbonate sediments, it has not been previously utilized for Mg/Ca paleothermometry. Cleaning experiments with synthetic composite samples (reagent CaCO3 powder and organic matter from noncalcifying marine algae Chlorella) are used to evaluate which traditional and/or novel cleaning methods permit us to recover the known carbonate Mg/Ca ratio. The most effective cleaning treatment, causing the least effect on carbonate chemistry and most complete and rapid oxidation of algal organic matter, was oxidation in an equal volume mixture of sodium hypochlorite (2.8%) and hydrogen peroxide (30%). However, in some organic- rich samples this method may not remove sufficient noncarbonate Mg to permit precise determination of carbonate Mg/Ca. Mg/Ca ratios in small culture samples may be determined precisely (relative standard deviation of 0.65%) using sector field inductively coupled plasma-mass spectrometry. Measurements of Mg/Ca in coccoliths from several species suggest that temperature may be an important control over Mg partitioning in coccolith calcite, although cleaning issues are likely to be an important limitation on paleoceanographic application.