13C double-quantum filtered correlation spectroscopy (DQF-COSY) provides a novel method for the detection of reactions involving carbon-bond scissions. We report the use of this technique to investigate isotopic exchange reactions of the non-oxidative pentose phosphate pathway in human erythrocytes. These exchange reactions resulted in the formation of a range of isotopic isomers (isotopomers) of glucose 6-phosphate after incubation of a mixture of universally 13C-labelled and unlabelled glucose 6-phosphate with fructose 1,6-bisphosphate and haemolysates. These isotopomers were detected in the coupling patterns of cross-peaks within the DQF-COSY spectrum of the deproteinized sample. A computer model which fully describes the reactions of the non-oxidative pentose phosphate pathway in human erythrocytes has previously been constructed and tested with 31P n.m.r. time-course data in our laboratory. This model was refined using 13C n.m.r. time-course data and extended to include the range of isotopomers which may be formed experimentally by the reactions of the non-oxidative pentose phosphate pathway. The isotopomer ratios obtained experimentally from the DQF-COSY spectrum were consistent with simulations generated by this model.