The development of two phosphate biosensors is described and compared for potentiometric detection of phosphate. Purine nucleoside phosphorylase (PNP) and xanthine oxidase (XOD) were co-immobilised by chemical cross-linking with glutaraldehyde (GLA) and bovine serum albumin (BSA), and via entrapment into polypyrrole (PPy) films by galvanostatic polymerisation. The BSA-GLA film was made with 4.5% v/v GLA and 6.8% w/v BSA with a drying time of 30 min, while polypyrrole entrapment was achieved with 0.5 M pyrrole by using a polymerisation time of 200 s. A mole ratio of 1:8 (6.2 U/mL XOD: 49.6 U/mL PNP) was used for both methods of enzyme immobilisation. Sensitive potentiometric measurements obtained for phosphate with the BSA-GLA-PNP-XOD biosensor were compared with those of PPy-PNP-XOD-Fe(CN)6 4- biosensor. A minimum detectable concentration of 0.1 mg/L phosphate and a linear concentration range of 0.5-2.5 mg/L were achieved with the PPy-PNP-XOD-Fe(CN)6 4- biosensor. In comparison, a minimum detectable concentration of 2 mg/L and a linear concentration range of 4-12 mg/L were achieved with the BSA-GLA immobilisation. The presence of uric and ascorbic acids had the least effect on the performance of the PPy-PNP-XOD-Fe(CN)6 4- biosensor, but will not have any effect on phosphate measurement with both biosensors at levels normally present in water.