No matter how often or thoroughly you clean your thermos, there's always the chance of mold getting in there. This doesn't mean it's curtains for your favorite thermal drink container: Cleaning the mold out of a thermos is a cakewalk, and it shouldn't take you more than a few minutes.
Rinse the thermos with hot water to keep the mold wet. This will prevent spores from flying into the air while cleaning and will make the mold easier to remove.
Mix a little bit of bleach with water and let it sit in the thermos for five to 10 minutes. This will kill the mold.
Empty out the bleach/water solution and rinse out the thermos.
Wash the thermos with dish detergent and water, using a sponge or scrub brush to scrape away any mold that's still inside the thermos.
Dry the thermos thoroughly, making sure there's no leftover mold. Your thermos is ready to use again.