The phase behavior and surface tension of aqueous, linear polyvinylamine (PVAm) bearing pendant phenylboronic acid moieties (PBA) were measured as functions of pH and the degree of PBA substitution. Coupling of PBA with 4% of the nitrogen on low molecular weight (15 kDa) PVAm gave water-soluble copolymer from pH 1 to 12. By contrast, derivatives of 150 kDa PVAm phased separated at pH 9 with as little as 4% derivatization. The pH ranges over which phase separation occurred increased with PBA substitution. The copolymer based on 51% substitution was insoluble over most of the pH range. Electrophoresis and potentiometric titration confirmed that the copolymers were amphoteric with isoelectric points around pH 9.5. The hydrophobicity of the copolymers was reflected in the significant lowering of surface tension, particularly at high pH. Finally fructose, which binds to borate, influenced the titration curves but did not influence surface tension.