Molar absorptivities have been assigned to VI4(g) for the wavelength interval 230-500 nm. Equilibrium and thermodynamic constants in the range 550-840 K are derived from absorbance data for the following reactions (standard state 1 bar) : (1) 2VI3(s) reversible 2VI2(s) + I2(g) (DELTAH-degrees/kJ.mol-1) = 93.0(+/-0.8), DELTAS-degrees-/(J.mol-1.K-1) = 113.5(+/-1.3)); (2) VI2(s) + I2(g) reversible VI4(g) (DELTAH-degrees/(kJ.mol-1) = 59.4(+/-0.7), DELTAS-degrees/(J.mol-1.K-1) = 43.4(+/-1.0)); (3) 2VI3(s) reversible VI2(s) + VI4(g) (DELTAH-degrees/(kJ.mol-1) = 152.3(+/-2.9), DELTAS-degrees/J.mol-1.K-1) = 156.8(+/-4.5); (4) VI3(2) + 0.5I2(g) reversible VI4(g) (DELTAH-degrees/(kJ.mol-1) = 104.8(+/-1.8), DELTAS-degrees/(J.mol-1.K-1) = 98.4(+/-2.7).