Reaction of single crystals of benzoic and trans-cinnamic acids with 200Torr pressure of ammonia gas in a sealed glass bulb at 20°C generates the corresponding ammonium salts; there is no sign of any 1:2 adduct as has been reported previously for related systems. Isotopic substitution using ND 3 has been used to aid identification of the products. Adipic acid likewise reacts with NH 3 gas to form a product in which ammonium salts are formed at both carboxylic acid groups. Reaction of 0.5Torr pressure of NO 2 gas with single crystals of 9-methylanthracene and 9-anthracenemethanol in a flow system generates nitrated products where the nitro group appears to be attached at the 10-position, i.e. the position trans to the methyl or methoxy substituent on the central ring. Isotopic substitution using 15 NO 2 has been used to confirm the identity of the bands arising from the coordinated NO 2 group. The products formed when single crystals of hydantoin are reacted with NO 2 gas under similar conditions depend on the temperature of the reaction. At 20°C, a nitrated product is formed, but at 65°C this gives way to a product containing no nitro groups. The findings show the general applicability of infrared microspectroscopy to a study of gas–solid reactions of organic single crystals.