Tungsten oxide thin films prepared by reactive magnetron sputtering are investigated for their use in infrared reflectance modulation electrochromic devices. The temperature dependence of the charge capacity and associated reflectance modulation are studied for W oxide films deposited in the temperature range 100-350 degreesC. Firms were cycled in either 1 M lithium triflate in propylene carbonate or 1 M H2SO4 electrolytes. The charge capacity of films of thickness similar to 200 nm is observed to decrease with increasing deposition temperature from similar to 50 to less than 25 mC cm(-2) for films prepared at the highest temperatures. Infrared spectral optical properties were measured and the optical constants, n(lambda) and k(lambda), then derived using an optical model. For all films prepared at 200 degreesC and above the optical constants are increased by ion insertion. The dependence of n(lambda) and k(lambda) on the quantity of charge inserted for films prepared at different substrate temperatures is established. The data enabled the design of variable reflectance devices utilising either the charge dependence of n(lambda) or of k(lambda). Examples of such devices prepared experimentally are presented.