We have investigated the reliability of 2D-COSY and 2D-TOCSY experiments to provide accurate measurements of (13)C-enrichments in complex mixtures of (13)C-labelled metabolites. This was done from both theoretical considerations and experimental investigations. The results showed that 2D-TOCSY but not 2D-COSY could provide accurate measurements of (13)C-enrichments, provided efficient zero-quantum filters were applied during the mixing period. This approach extends the range of NMR methods applicable in (13)C-labelling experiments and is suitable to investigating the dynamic behaviour of metabolic systems.