Originally Posted by trooper

Just to emphasise. You can't look at awards on Air Canada (priced in THEIR miles/points ) and pay with United miles. (Think of them as different currencies!) You CAN, as A Little Cow said, use United miles to purchase AC flights... but that must be through United. I do not know if United has a part miles/part cash payment option but if you don't have enough miles alone to redeem flights that might be worth checking. OR perhaps (assuming it is TWO seats you want) look at ONE award and one paid?

Originally Posted by A Little Cow

You can't use United miles on Air Canada's website, but you can use them for partner flight (including AC) mileage purchases if there is award availability via united.com. How many United miles do you have, how many people are flying, and how flexible are you with timing?