Photomask plasma etch reactions were studied using Gibbs energy minimization method. The equilibrium compositions were analyzed at practical photomask plasma etch conditions of temperatures, pressures, and reactant inputs. The thermodynamic calculations were based on common gaseous systems used in photomask plasma etching such as Cl2-O2-He, SF6-O2-He, and CF4-O2-He, as well as alternative gases. For Cr etch, the thermodynamically calculated results showed that volatile CrO2Cl2 was the moderate equilibrium composition in the predetermined system only when the temperature was higher than 400°C, indicating that temperatures of heavy particles in practical plasma conditions might be higher than this temperature. The effects of assistant chemicals on equilibrium compositions were investigated. For MoSi etch, the thermodynamic calculation showed that the main volatile etch products were MoF6 and SiF4. The comparison of MoSi etch using SF6 and CF4 was made and gaseous input condition for obtaining all volatile products was found, which would be helpful for defectivity and passivation controls. The calculation also showed that the addition of oxygen in SF6 and CF4 systems could increase the equilibrium composition of atomic fluorine, resulting in the etch rate increase. This result agreed with previous hypothesis on the oxygen effects on etch rate. For quartz etch, the calculation showed that the main volatile etch product was SiF4. For Ta or TaN absorber EUV mask etch, the volatile Ta-containing product was found to be TaCl5.