Optical polarization by water-ice frost formed at temperatures down to 100 K is measured, and found to be significantly different from that previously observed for terrestrial ice and frost; the difference is greatest for frost with very small - possibly subwavelength - ice crystals, formed by very rapid cooling of a small amount of water vapour: this is found to give deep negative polarization at a small phase angle, and a high value of slope. Assembled ice and frost data give a new slope/albedo relationship, significantly different both from the well-established relationship for terrestrial and planetary rocks, and from the intermediate band for opaque lightcoloured powders. This new relationship is relevant to the interpretation of observations of icy planetary satellites. The UMIST polarimeter, with its lowtemperature sample holder, is described. Key words: polarization - instrumentation: polarimeters - planets and satellites: general.