High surface area molybdenum nitride (gamma -Mo2N) and two kinds of carbides (alpha -MoC1-x or beta -Mo2C) were prepared by the temperature programmed reaction with NH3 for the former sample, and with CH4 + H-2 mixed gas for the latter samples. To prepare gamma -Mo2N, the precursor MoO3 was heated up to 973 K and held for 2h under NH3 flow. To prepare alpha -MoC1-x, the precursor gamma -Mo2N was heated up to 973 K and held for 2h under CH4 + 4H(2) flow. To prepare beta -Mo2C, the precursor MoO3 was heated up to 973 K and held for 2h under CH4 + 4H(2) flow. The surface areas of gamma -Mo2N and alpha -MoC1-x were about 140 m(2) g(-1); on the other hand, that of beta -Mo2C was 32 m(2) g(-1). However, beta -Mo2C was most active among those samples and was as active as the doubly-promoted iron catalyst. The stability of those catalysts treated with the reactant gas at 873 K was examined under the reaction condition. The activity of alpha -MoC1-x was decreased because this phase was not stable. C-N atomic exchange occurred. gamma -Mo2N and beta -Mo2C were more stable than alpha -MoC1-x. To improve the catalytic activity, alkali was added. Addition of caesium did not affect the formation of gamma -Mo2N and alpha -MoC1-x, and the activities were somewhat improved. On the other hand, Mo metal was formed instead of beta -Mo2C by the carburization from MoO3 with Cs added.