The present invention relates to the preparation of powdered heat-curable reaction mixtures in which melt suspension of (a) solid polyisocyanates having melting points above about 60.degree. C., (b) molten high molecular weight isocyanate-reactive compounds having molecular weights of from 400 to 10,000 that have melting or softening points of 50.degree. C. to 150.degree. C., and (c) optionally, solid or liquid low molecular weight chain-extending or crosslinking compounds having molecular weights of from 62 to 399, are added to an inert solvent containing an emulsifier, thereby forming powdered heat-curable reaction mixtures in which components (a), (b), and (c) remain in uncrossedlinked form. The invention further relates to the use of the heat-curing reaction mixtures for the preparation of high-quality polyurethane polyurea plastics.