distilled water is ok BUT... what your actually suposed to use is Deionized water, also known as demineralized water or DI water (its distilled water put through a chemical filter process). if you know anybody that works at a lab or chemical plant they can easily get some for you. if you dont, check water purification shops. this isnt esential but it is highly recomended.MC said:It was also recomended to me to use distilled water with the antifreeze for the 50/50 mix. It is supposed to help further prevent corosion and build up inside the cooling system. A friend of mine that has been working bikes for about 35 years is the one who told me about it so I took his word. Done this with both bikes I have owned and pretty satisfied.