Experimental adsorption kinetics and equilibrium results of penicillin G acylase (PGA, from Escherichia coli, EC 3.5.1.11) on mesoporous silicas with pore sizes ranging from 5.6 to 33.2 nm showed that samples with pore sizes between 11.0 and 13.2 nm exhibited the best performance in immobilizing PGA under mild experimental conditions. A mesoporous silica sample with an optimum pore size of about 11.5 nm was then modified with different amounts of glycidoxypropylmethoxysilane to yield oxirane-functionalized silicas of different densities of surface oxirane groups. Under very mild incubation conditions, a partially oxirane-functionalized silica sample was found to be more efficient in immobilizing PGA than a fully oxirane-functionalized sample and a commercial polymer carrier (i.e., Eupergit C). With the partially oxirane-functionalized mesoporous silica sample as a carrier, a PGA loading of 110 mg/g (dry support) and an enzymatic activity of as high as 3477 unit/g (dry support) were achieved within 24 h of incubation. The residual surface silanol groups on the partially oxirane-functionalized silica were observed to play a pivotal role in facilitating the covalent binding of PGA with the oxirane groups at low salt concentrations.