That is MPS. It seems most bottles list the chemical name that way, so I'm not clear on why people refer to it as MPS on these boards, other than to avoid initials commonly associated with "women's problems". My bottle of "Non-Chlorine Shock" says the same thing but with 45% MPS, so I'm guessing yours is buffered so it doesn't affect pH so much, or contains some other goodies like a little dichlor or clarifier.