Nylon tube O-alkylation and its effects on the catalytic efficiency of urease (E.C. 3.5.1.5) immobilised on the tube were examined using dimethyl sulphate (DMS) and diethyl sulphate (DES) as alkylating agents on Nylon 6 of different wall thicknesses. Effective and easy control of the alkylation process was possible as both reagents scarcely reacted with nylon at room temperature (25 °C) and as high-temperature alkylation, at 100 °C, could be stopped rapidly by immersing the tube in an ice-bath. The optimum incubation times for alkylation with DMS and DES were 3 and 10 min, respectively. Diethyl sulphate, a less toxic reagent than DMS, caused less damage to the nylon tubes and produced more chemically stable O-alkylated derivatives. However, it was less efficient than DMS in creating reactive sites for covalent attachment of catalytically active urease on the tube. Although thick-walled nylon tubes immobilised more active enzyme, such tubes were less pliable than thin-walled tubes and could pose operational problems.