We have studied the capillary condensation of water from saturated vapor below 0 degrees C in the annular wedge-pore formed around two mica surfaces in contact in a surface force apparatus. The condensed water remains liquid down to at least -9 degrees C, and the measured condensate size is close to the predictions of it recent model for the dependence of the interfacial curvature of supercooled capillary condensates on temperature and surface tension. The small deviation observed may be accounted for by assuming that solute as K2CO3 from the mica-condensate interface dissolves in the condensates and gives rise to an additional depression of the freezing point apart from that caused by the interface curvature. Ay contrast, measurements of the interface curvature at relative vapor pressures of 0.95-0.99 at 20 degrees C confirm it significantly larger deviation from the Kelvin equation. The magnitude of the deviation is in remarkable agreement with that calculated from the results of an earlier study of capillary condensation of water from a nonpolar liquid, also at T = 20 degrees C. Evidently, additional solute from the surrounding mica surface migrates into the condensates at room temperature. We conclude that the surface diffusion of ions on mica is much slower et subzero temperatures than at room temperature.