Molybdenum nitride and carbide containing phosphorus were prepared by temperature programmed reaction (TPR) of 12-molybdophosphoric acid, a Keggin-type heteropolyacid (HPA), with NH3 and CH4/H-2. The process of solid transformation and the properties of prepared materials were compared with those of nitride and carbide made from MoO3. Product phases were produced at much lower temperature from HPA, and smaller crystallite sizes were obtained. Yet, HPA-derived materials showed lower surface areas and CO chemisorption probably because phosphorus remaining in the products blocked their pores and adsorption sites. Since the involved solid transformations were topotactic, two precursors yielded products with completely different morphology of the particles. The distribution of phosphorus in HPA-derived Mo2N was highly uniform.