Immobilization of biomass by poly-vinyl-alcohol (PVA) encapsulation is an effective alternative for advanced wastewater treatment. The application of PVA immobilization has been gaining importance in the last few decades, but little information is available about the effect of PVA characteristics on physical properties as an encapsulation media. This study investigated the effect of PVA characteristics (such as saponification ratio and molecular weight) on the solubility of the immobilized PVA gels. In addition, the effect of filler-addition on the solubility of PVA was investigated too. As the saponification ratio (SR) of PVA increased, the solubility decreased. The solubility of the PVA gels decreased as the molecular weight (MW) of PVA increased for the fully saponificated PVAs (F series), whereas the solubility of the partially saponificated PVAs (P series) decreased as the molecular weight decreased. These phenomena could be explained by inter- and intramolecular hydrogen bonding between PVA molecules. The addition of powdered activated carbon (PAC) to PVA lessened the solubility significantly. For example, the solubility of P24 was reduced to 62%. In the case of immobilization with biomass (both of activated sludge and nitrifiers), the overall solubility tendency of each PVA was not changed (P24 > P17 > F05 > F17). (c) 2005 Elsevier Ltd. All rights reserved.