γ-Butyrobetaine hydroxylase (BBOX) is a 2-oxoglutarate (2OG) dependent oxygenase that catalyses the stereoselective C-3 hydroxylation of γ-butyrobetaine (GBB) to give L-carnitine. L-Carnitine is involved in fatty acid metabolism in all animals and in some prokaryotes, and BBOX is a current drug target for the treatment of myocardial infarction. We describe the development and application of ¹H NMR GBB/2OG reporter based assays employing paramagnetic relaxation enhancement to monitor inhibitor binding to the BBOX active site. In a single experiment, the method assesses inhibitors for competitive binding with 2OG or GBB, or both. The method was exemplified with a set of isoquinoline-based inhibitors; the results reveal structure–activity relationships that were not predicted from crystallographic studies, with some inhibitors competing 2OG only and some competing both 2OG and GBB. The method will also be applicable to work on the inhibition of other 2OG oxygenases.