The application of membranes to the enzymatic production of structured lipids has been investigated using an ultrafiltration membrane reactor for a reaction between medium chain triacylglycerols (MCT) and n-3 polyunsaturated fatty acids from fish oil. Lipozyme IM was used as the biocatalyst. The incorporation of polyunsaturated fatty acids into MCT was increased by about 15% over 80 h by simultaneous separation of the released medium chain fatty acids compared to control.