Poly(vinyl alcohol) (PVA), a hydrophilic polymer bearing hydroxyl functional groups, readily forms gels on the addition of a bifunctional agent such as glutaraldehyde. This gelation is caused by the formation of acetal bonds between the aldehyde groups and the hydroxyl groups, in the presence of HCl. The crosslink density is easily controlled by changing the concentration of glutaraldehyde, offering a way to control the total water content and, consequently, the permeability. Strong transparent films were obtained by drying these gels. These were suitable for supporting biologically active molecules, providing a viable, better alternative to the usual PVA gels which are physically crosslinked. To establish the relationship between their thermal properties and the occurrence of the physical or chemical gelling, their water vapor sorption and thermal behavior were investigated and compared. PVA/glutaraldehyde membranes were prepared with immobilized hemoglobin for bilirubin analysis. Assisted by the respective equilibrium swelling ratio and thermal stability data, it was found that, for these particular PVA/hemoglobin membranes, the occurrence of either chemical or phyiscal crosslinking is mainly determined by the Hb/HCl ratio.