No, no, no! Bleach cannot be used to clean a stainless steel thermos. Thermal flasks can be highly damaged by bleaching agents, so you should avoid them at all costs. It is important to check your thermos' cleaning instructions, as some thermoses can be washed in the dishwasher. You can remove any stains and odors from your thermal flask using a combination of vinegar, baking soda, and hot water. Taking care of your coffee thermos is easy if you follow these steps.