The quality of steel produced by continuous casting is strongly dependent on the performance of mold powders. Mold powders govern the steel production in terms of production rate, cleanliness, surface condition, defects and environment of casting operation. Melting behaviour of mold powders is one of the most important performance parameters for mold fluxes and is mainly dependent on carbon, which is an essential constituent of mold fluxes. The following study establishes the effect of carbon structure and morphology on the melting behaviour of mold fluxes. This has been achieved by characterizing carbon of its structure and reactivity and relating it to the melting rate of mold powders. The reactivities of carbons were studied in a new light. Carbons are considered to be composed of two materials-amorphous carbon and crystalline carbon and hence they should possess two separate activation energies. Carbon blacks have lately attracted considerable attention for use in mold powders due to fine size. As a result a study was conducted to see the effect of particle size of various carbon blacks on the melting rate of mold powders. The melting behaviour is also studied as a function of composition by using the concept of optical basicity.