A new technique for determining sea surface temperatures (SST), based on the unsaturation ratio, U 37 k , of long chain C 37 methyl alkenones produced by the phytoplankton Prymnesiophyceae, has previously been shown by other researchers to have a linear response with temperature in culture and water column studies, and a good correlation with δ 18O over ∼ 100,000 years in a core from the east Equatorial Atlantic. These results indicate this technique has potential for determining paleo-SST in marine sediments. In order to apply U 37 k rigorously and quantitatively in sediments for paleotemperature estimations, we have calibrated the method in sediments and evaluated how well the alkenone's temperature signal is preserved under some common conditions of deposition and sample handling, i.e. the effects on these compounds of dissolution of sedimentary carbonates, and storage and sampling. Comparison of U 37 k in duplicate samples collected and stored frozen versus those stored at room temperature for up to four years shows no significant differences. Carbonate dissolution which renders foraminifera-based techniques unusable or unreliable show no effect on U 37 k values in laboratory experiments. Initial field results support this, but more studies are necessary. The U 37 k “thermometer” was calibrated by analyzing U 37 k in core-tops from widely varying open ocean sites. Sediment values of U 37 k reflected overlying SST, with the results falling on the same regression line for culture and water column samples of Prahl and Wakeham [1], indicating that their equation ( U 37 k = 0.033 T + 0.043) is suitable for use in converting U 37 k values in sediments to overlying warm season SST. Neither a wide range of productivity nor lowered salinity of the overlying waters appears to affect U 37 k accuracy. With a calibration for sediments, the U 37 k method can begin to be quantitatively applied down core to open ocean sediments for paleotemperature estimations of the late Quaternary.