It's a concentrated syrup. The exact formula is proprietary but based on the published patents and MSDS the developing agents are Phenidone (or one of the Phenidone derivatives) and Hydroquinone. It is a traditional favourite of Zone System users, mostly because Adams was using it in the 70s/80s, and because it is easily diluted to different working strengths. It also last a long time. The working characteristics are very similar to D-76.