The aim of the investigations was to synthesise photocrosslinkable hydrophobic prepolymers based on poly(vinyl alcohol) for the use in enzyme entrapment. Therefore, the influence of the sequence of acetalisation of poly(vinyl alcohol) with a photocrosslinkable aldehyde 1-methyl-4-[2-(4-formylphenyl)-ethenyl]sulphate (SbQ-4) and propionaldehyde was examined to find out the reaction conditions yielding prepolymers with similar content of photocrosslinkable component but different amounts of propionaldehyde acetals. Data obtained from quantitative C-13-NMR spectroscopy and UV spectrophotometry proved that a stepwise acetalisation first with SbQ-4 leads to the desired prepolymers. In contrast, during simultaneous acetalisation competition of both aldehydes yields prepolymers with a content of photocrosslinkable group that depends on the degree of acetalisation with propionaldehyde.