I use them on the tops all the time, usually between the shoe and the bootie. They aren't as efficient warming your toes but they do work there. We do the same thing when XC skiing, the warmer goes on the top of the boot over the toes before the boot cover goes on. Make sure they are already warm before doing this, sometimes on the ski boots you won't get enough air to get them started but once they are going they seem to keep going.