Porous nickel oxide films were deposited onto unheated indium tin oxide coated glass substrates by reactive dc magnetron sputtering. These films had a cubic NiO structure. Electrochromic properties were evaluated in 1M potassium hydroxide (KOH) and in 1M lithium perchlorate in propylene carbonate (Li–PC). Large optical modulation was obtained for ~500-nm-thick films both in KOH and in Li–PC (~70% and ~50% at 550nm, respectively). In KOH, tensile and compressive stresses, due to the expansion and contraction of the lattice, were found for films in their bleached and colored state, respectively. In Li–PC, compressive stress was seen both in colored and bleached films. Durability tests with voltage sweeps between −0.5 and 0.65V vs Ag/AgCl in KOH showed good durability for 10,000cycles, whereas voltage sweeps between 2.0 and 4.7V vs Li/Li+ in Li–PC yielded significant degradation after 1000cycles.