[header] How to clean rubber bath mats [title] Prepare a diluted bleach solution in a bathtub. [step] If this is the tub that contains the bath mat, lift up the bath mat so it isn't stuck to the bottom of the tub. An easy and effective way to clean your rubber bath mat is to soak it in a diluted bleach solution in your bathtub. 
If you have a deep or utility sink, that may be preferable. Be prepared to measure the amount of bleach to be used in making your solution and have more than enough room for a proper soaking.