I tried diluted alcohol, diluted simple green, nada, and I don't want to use anything petroleum on the rubber keys. Then it occur to me, the cleanser I use for my contact lens may work. IT WORKED! I also use this for my earphone's tips, they are removable. I figure anything safe for my eyes ought to be safe for the keys. Suggested Procedure: Damp q-tip with the liquid, but not too wet! you don't want the liquid to get into the cracks, dap the affected keys, let it sit for a couple of minutes, but don't let it dry! If u applied too much, use another dry q-tip to soak up the excess. Then rub it, use another q-tip to make sure everything is dry. If your stains is been sitting around for a while, it may take several tries. As usual, your mileage may vary.