We have performed a series of molecular dynamics simulations of alkali metal (Li+, Na+, K+, Rb+, and Cs+) and halide (F-, Cl-, Br-, and I-) ions in water at infinite dilution at T=258 K to investigate the effects of ion size on the hydration structure and diffusion of ions in supercooled water. Simulations are also performed at T=298 K in order to compare the results of the hydration structure and diffusion in supercooled water with those in ambient water. With increase of ion size, like in ambient water, in supercooled water also the diffusion coefficients of alkali metal and halide ions are found to fall in different curves with distinct maxima. However, the relative increases of the diffusion coefficients of larger ions compared to those of Li+ and F- are found to be significantly higher in the supercooled water. (C) 2003 American Institute of Physics.