The stereochemical course of the recently isolated fluorination enzyme from Streptomyces cattleya has been evaluated. The enzyme mediates a reaction between fluoride ion and S-adenosyl-L-methionine (SAM) to generate 5′-fluoro-5′-deoxy-adenosine (5′-FDA). Preparation of (5’R)-[5-H-2(1)]-ATP generated (5′-R)-[5-H-2(1)]-5′-FDA in a coupled enzyme assay involving SAM synthase and the fluorinase. The stereochemical analysis of the product relied on H-2 NMR analysis in a chiral liquid-crystalline medium. It is concluded that the enzyme catalyses the fluorination with an inversion of configuration consistent with an S(N)2 reaction mechanism.