Question: does 20 volume developer expire?
Answer: Peroxides or Developers don't 'spoil' or go bad but they can deteriorate meaning the Hydrogen Peroxide breaks down to free oxygen and water causing the product to weaken (go from 10 Vol. to 9 Vol. or less) or it can dehydrate or lose water and get more 'concentrated' (go from 20 Vol. to 22 vol. or more) However, all ...