The economic viability of biocatalytic conversions is often dependent on finding an effective method for immobilization of the enzyme involved. This provides for its improved operational stability and facile recovery and re-use. Cross-linked enzyme aggregates (CLEAs®) constitute an effective methodology for enzyme immobilization with broad scope. The technique is exquisitely simple, involving precipitation from aqueous buffer and subsequent cross-linking of the resulting physical aggregates of enzyme molecules, and amenable to rapid optimization. The resulting CLEAs are stable, recyclable biocatalysts exhibiting high activity retention, in some cases higher than that of the free enzyme they were derived from. The enzyme does not need to be of high purity since the methodology essentially combines purification and immobilization into a single operation. The technique can also be applied to the preparation of combi-CLEAs containing two or more enzymes. For example, an oxynitrilase/nitrilase CLEA for the one-pot synthesis of (S) mandelic acid from benzaldehyde in high yield and enantioselectivity.