A simple, rapid, and quantitative NMR method was developed for the determination of the ratio of the diastereoisomers present in 7-chloro-3,3a-dihydro-2-methyl-2H, 9H-isoxazolo(3,2-b)(1,3)benzoxazin-9-one (I). Deuterated tris(1,1,1,2,2,3,3heptafluoro-7,7-dimethyl-4,6-octanedionato)europium I11 shift reagent causes the doublet assigned to the protons of the 2-methyl group, which normally appear a t about 1.5 ppm, to split into a pair of doublets and shifts them downfield to about 3 ppm. Each new doublet represents one racemic diastereoisomeric pair present in I. The ratio of the pairs was determined by NMR integration of the areas under the respective doublets by calculating their proportions relative to the total area under the doublets. When a series of prepared mixtures of the isolated pairs was analyzed and their ratios were determined, the results obtained were within I% of their actual values. Keyphrases 7-Chloro-3,3a-dihydro-2-methyl-2H,9H-isoxazolo-(3,2-b)(1,3)benzoxazin-9-one diastereoisomers-NMR analysis, deuterated tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)europium I11 shift reagent Shift reagents-deuterated tris( 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)europium I11 for NMR analysis of diastereoisomers of 7-chloro-3,3adi hydro-2 -methyl-2H, 9H-isoxazolo( 3,2-b) (1,3)benzoxazin-9-one Tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethy~-4,6~octanedionato)europium I11 shift reagent-NMR analysis of 7-chloro-3,3adihydro-2-methyl-2H, 9Hisoxazolo(3,2-b)(1,3)benzoxazin -.9one diastereoisomers NMR-analysis, 7-chloro-3,3a-dihydro-2methyl-2H,9H-isoxazolo(3,2-b)(1,3)benzoxazin-9-one diastereoisomers