This study deals with the thermodynamic equilibrium for xylene isomerization. Experiments performed by several researchers to calculate the equilibrium in the gas phase in the 1990s led to the conclusion that the earlier available thermodynamic data for xylenes, which were mainly based on experimental work performed in the 1940s, were in error. In this work a similar procedure was followed to determine the thermodynamic equilibrium for xylene isomerization in the liquid phase. By means of the thermodynamic functions at saturated conditions presented by the previously mentioned studies, the standard free energies of formation were calculated between 250 K and 550 K. Three different expressions were developed to calculate the equilibrium constants as a function of temperature.