Thermodynamic phase equilibria in synthetic slags (Al2O3-CaO-FeO-SiO2-V2O3) were investigated with 0-27 wt.% vanadium oxide corresponding to industrial coal-petroleum coke (petcoke) feedstock blends in a simulated gasifier environment. Samples encompassing coal-petcoke mixed slag compositions were equilibrated at 1500 degrees C in a 64 vol.% CO/36 vol.% CO2 atmosphere (Po-2 approximate to 10(-8) atm at 1500 degrees C) for 72 h, followed by rapid water quench, then analyzed by inductively coupled plasma optical emission spectrometry, X-ray diffractometry, and scanning electron microscopy with wavelength dispersive spectroscopy. With increasing CaO content, FeO content, or both; the slag homogeneity region expanded and a composition range exhibiting crystals was reduced. The mullite (AI(6)Si(2)O(13)) crystalline phase was not present in the slags above 9 wt.% FeO while the karelianite (V2O3) crystalline phase was always present in compositions studied if a sufficient amount of vanadium existed in the slag. Based on the present experimental equilibrium evaluation, a set of isothermal phase diagrams showing effects of CaO and FeO on thermodynamic phase stabilities in the vanadium-bearing slags is proposed. Some uses of the diagrams for potential industrial practice are discussed. Published by Elsevier Ltd.