ABSTRACT Hydroxynitrile lyase from Arabidopsis thaliana ( At HNL) was fused to different fluorescent reporter proteins. Whereas all fusion constructs retained enzymatic activity and fluorescence in vivo and in vitro , significant differences in activity and pH stability were observed. In particular, flavin-based fluorescent reporter (FbFP) fusions showed almost 2 orders of magnitude-increased half-lives in the weakly acidic pH range compared to findings for the wild-type enzyme. Analysis of the quaternary structure of the respective FbFP- At HNL fusion proteins suggested that this increased stability is apparently caused by oligomerization mediated via the FbFP tag. Moreover, the increased stability of the fusion proteins enabled the efficient synthesis of ( R )-mandelonitrile in an aqueous-organic two-phase system at a pH of <5. Remarkably, ( R )-mandelonitrile synthesis is not possible using wild-type At HNL under the same conditions due to the inherent instability of this enzyme below pH 5. The fusion strategy presented here reveals a surprising means for the stabilization of enzymes and stresses the importance of a thorough in vitro characterization of in vivo -employed fluorescent fusion proteins.