The utilization of cofactor-dependent redox enzymes in bioprocess technologies requires low cost cofactor regeneration methods. PEGylated NAD(H) (PEG-NAD(H)) has been utilized in enzyme membrane reactors as a means to recover the cofactor; however, there is a lack of understanding of the effect of PEGylation on enzymatic activity, especially on the relationship between biocatalysis and transport phenomena. To explore this further, two redox enzymes (formate dehydrogenase (FDH) from Saccharomyces cerevisiae and NAD(H)-dependent d-lactate dehydrogenase (nLDH) from Escherichia coli) have been chosen and the kinetic effects caused by cofactor modifications (with PEG of three different chain lengths) have been investigated. The PEGylation did not impact the cofactor dissociation constants and mass transfer was not the rate-limiting step in biocatalysis for either enzyme. However, the PEG chain length had different impacts on the formation of enzyme/cofactor and/or enzyme/cofactor/substrate ternary complexes for the enzymes. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 12-17, 2018