A new zwitterionic monomer 3-[diallyl{3-(diethoxyphosphoryl)propyl}ammonio]propane-1-sulfonate has been synthesized and cyclopolymerized to give the corresponding polyzwitterion (+/-) (PZ) bearing both phosphonate and sulfonate functionalities on each repeating unit. phosphonate ester hydrolysis in PZ gave a pH-responsive dibasic polyzwitterionic acid (+/-) (PZA) bearing PO3H2 units. The PZA under pH-induced transformation was converted into polyzwitterion/anion (+/- -) (PZAN) and polyzwitterion/dianion (+/- =) (PZDAN) having respective PO3H- and PO32- units. The polymers interesting solubility and viscosity behaviors have been investigated in detail. The apparent protonation constants in salt-free water and 0.1 M NaCl of the PO32- in (+/- =) (PZDAN) and PO3H- in (+/- -) (PZAN) as well as in their corresponding monomeric units have been determined. Evaluation of antiscaling properties of the PZA using supersaturated solution of CaSO4 revealed approximate to 100% scale inhibition efficiency at a meager concentration of 20 ppm for a duration of 45 h at 40 degrees C. The PZA has the potential to be used effectively as an antiscalant in reverse osmosis plant. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 5130-5142