The {100} platelets of a type IaA/B diamond have been examined by spatially resolved electron-energy-loss spectroscopy. It is demonstrated unambiguously that nitrogen is present at these planar defects but in low concentrations. A quantitative analysis of the data reveals that the atomic density of the nitrogen is 0·23 (±0·14)atoms pera20, that is about one tenth of a monolayer. This amount is significantly less than predicted by the nitrogen-containing structural models. It is concluded that the platelets are most likely to be composed of carbon and contain nitrogen as a major impurity species.