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A facile and simple strategy to prepare ammonia gas-modified graphene oxide (GO) catalysts was successfully established by gas–solid acid–base reaction at room temperature. The catalytic performances of ammonia gas-modified GO samples were examined in Knoevenagel condensation. The samples were characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, X-ray photoelectron spectroscopy, atomic force microscope, NH3 temperature-programmed desorption and elemental analysis. The results indicated that the excellent performances of the ammonia gas-modified GO samples in Knoevenagel condensation should be ascribed to the formation of ammonium ions (NH4+) by the reaction between ammonia gas and the carboxyl groups located on the edge of the GO. [ABSTRACT FROM AUTHOR]