Knowledge about palaeo sea ice conditions in the Arctic Ocean and its marginal seas serves to constrain the natural variability in sea ice cover and how this relates to atmospheric and/or oceanic circulation changes. The biomarker IP25, a monounsaturated C25 highly branched isoprenoid exclusively produced by sea ice diatoms (Brown et al., 2014; Belt et al., 2007), is a relatively new tool for sea ice reconstructions. Other than calcareous or siliceous walled microfossils, IP25 exhibits a good preservation potential in the polar and subpolar high northern latitudes. Its reliability as a direct indicator of a former sea ice cover has been demonstrated within several studies conducted on Arctic Ocean surface sediments. A further approach aiming at a semi-quantitative assessment of palaeo sea ice coverage is the PIP25 index, which combines the information on sea ice occurrence (IP25) and phytoplankton productivity (e.g. through diatom or dinoflagellate biomarkers; Müller et al., 2011). Both IP25 and the PIP25 index have been used repeatedly for reconstructions of how Arctic sea ice changed through time. This presentation summarises the current knowledge on the source and distribution of IP25, discusses the use of the PIP25 index and provides examples of IP25/PIP25 based palaeo sea ice studies. References Belt, S.T., Massé, G., Rowland, S.J., Poulin, M., Michel, C., LeBlanc, B., 2007. A novel chemical fossil of palaeo sea ice: IP25. Organic Geochemistry, 38, (1), 16-27. Brown, T.A., Belt, S.T., Tatarek, A., Mundy, C.J., 2014. Source identification of the Arctic sea ice proxy IP25. Nature Communications, 5:4197. Müller, J., Wagner, A., Fahl, K., Stein, R., Prange, M., Lohmann, G., 2011. Towards quantitative sea ice reconstructions in the northern North Atlantic: A combined biomarker and numerical modelling approach. Earth & Planetary Science Letters, 306, (3–4), 137-148.