Molybdenum diselenides were synthesized at 1273 K for 24 h. The X-ray diffraction (XRD) analysis showed the compound to be non-stoichiometric in the approximate range of 1.85 less than or equal to Se/Mo less than or equal to 2.0. Density measurements suggested a co-existence of interstitial Mo and Se vacancies in the lattice over the nonstoichiometric range. Well-grown crystals of hexagonal 2Hb-type were formed in the range of 1.9 less than or equal to Se/Mo less than or equal to 2.0, whereas fine powders were given for Se/Mo < 1.9. In the latter range, a mixed-stacking of 2Hb- and 3R-type layers was suggested to take place, and the ratio of 2Hb:3R in the compounds of Se/Mo = 1.85 was estimated to be about 85:15 by means of a simulation of the XRD patterns. Compressed samples of 1.85 less than or equal to Se/Mo less than or equal to 1.9 were higher in the electrical conductivity and were smaller in its temperature dependence, compared with those of 1.9 less than or equal to Se/Mo less than or equal to 2.0.