<jats:title>Abstract</jats:title><jats:p>The chronology of Quaternary paleoenvironment and climate in northeastern Siberia is poorly understood due to a lack of reliable numerical age determinations. The best climatic archives are ice-rich permafrost sequences, which are widely distributed in northeastern Siberia. For this study, <jats:sup>230</jats:sup>Th/U-ages were determined by thermal ionization mass spectrometry (TIMS) from frozen peat in a permafrost deposit at the southern cliff of the Bol'shoy Lyakhovsky Island (New Siberian Archipelago), west of the Zimov'e River. These yielded a Pre-Eemian “isochron”-corrected <jats:sup>230</jats:sup>Th/U-age of 200,900±3400 yr. This result is reliable because permafrost deposits behave as closed systems with respect to uranium and thorium. Our findings suggest that <jats:sup>230</jats:sup>Th/U dating of frozen peat in permafrost deposits is a useful tool for the reconstruction of the Middle Quaternary environment of northern Siberia and of the whole Arctic.</jats:p>