β-ketoester-quinonemethide-carbonate blocked p-phenylenediamine color developers are reducing agents, but are 100 to 230 mV more resistant to oxidation than the free p-phenylenediamine (4-amino-3-methyl-N-ethyl-N-(β-methylsulfonamidoethyl)aniline). This resistance renders the blocked developer significantly less active than the free developer. Over pH 3 to 9 the oxidation of these blocked developers appears two electron, chemically reversible or quasi-reversible, and very analogous to the oxidation processes of the free developer. Chemical irreversibility at higher pH and at low pH is assigned to deamination of the quinonediimine. The sulfonamidophenol deriving from the quinonemethide timing group of the blocked developer is about 200 mV more resistant at pH 3 to oxidation than are the blocked developers and is slightly less (25 mV) resistant at pH 11. The oxidation of this sulfonamidophenol appears two-electron over pH 2 to 13.