We present observations of the transition in sea ice backscattering signatures from late summer into early winter in the Beaufort and Chukchi Seas during 1991 and 1992, using data from the ERS 1 synthetic aperture radar (SAR). We employ both analyzed surface temperature fields and direct observations of near-surface temperature from drifting buoys. Consistent with previous surface-based observations, backscattering from sea ice surviving at the end of summer increases strongly and rapidly when temperatures fall below freezing for the final time in the autumn, apparently because of increased volume scattering from bubbles in the upper part of the ice. Areally averaged backscattering sometimes overshoots typical winter multiyear ice values by 1--2 dB, before settling back over a period of approximately a week. The backscattering-temperature link forms a sufficient basis for an algorithm to estimate retrospectively, from time series of SAR images, the date of freeze-up (defined here to be the date on which all liquid water in the bubbly upper layer of surviving sea ice freezes and remains frozen for the duration of the ensuing autumn and winter). We present a prototype algorithm and use it to estimate freeze-up dates in nine Lagrangian cells in the Beaufort Sea during the autumn of 1992. We observe a 12-day spread in freeze-up dates between latitudes of approximately 73 ¿N and 82 ¿N, with dates in the northernmost cells of August 29--30 and those in the southernmost cells of September 7. Two cells at latitudes of 75 ¿N--77 ¿N appear to freeze-up earlier than cells further north but present uncertainties limit the significance of this observation. ¿ American Geophysical Union 1996