Entrained flow gasification occurs typically at temperatures above 1400 degrees C. Inorganic species present in feed-stocks like coal/petcoke melt and form slag. Slag viscosity is an important property for the design, operation and modelling the behavior of flows in gasifiers. Existing correlations between the composition and viscosity do not work accurately for petcoke ash slag due to V and Ni. In entrained flow gasifiers, the gaseous environment can affect the slag viscosity based on oxidation states of transition elements (primarily V, Fe and Ni). So, the mineral matter transformations need to be understood for the development of proper correlations. Coal ash can be adequately represented by the oxides of the following major elements: Si, Ca, Al, and Fe. In addition to those, petcoke ash contains significant amounts of V and Ni. The objectives of this work are to determine the effects of reducing atmosphere and pressure on the mineral matter transformations, and subsequently develop a slag correlation. CO-CO2 mixtures in different proportions were used to determine the effect of reducing atmosphere on the reduction of oxides of the transition elements. A high temperature X-Ray Diffraction (XRD) technique was used to study the transformations. A high-pressure thermogravimetric analysis (TGA) equipment was used to study the effect of pressure on the reduction reactions. XRD and TGA results indicated that V, Ni and Fe oxides were getting reduced. Consequently, lower oxidation state phases (V2O3, NiO and FeO) were used to develop slag viscosity correlation for a petcoke ash by linear regression. Viscosity data of melts with the aforementioned six oxides were obtained from the study of Wang et al. (2004). TGA results showed that the reduction reactions occurring at 1.01 bar, 10 bar and 20 bar contributed to mass loss of the synthetic petcoke ash. With increase in pressure, the reduction reactions occurred at higher temperature. X-Ray diffractograms indicated that CO concentration in the reactant gas influenced the transformations. Predictions made by the viscosity correlation with proper phases for V, Ni and Fe were in reasonably good agreement with experimental data.