The infrared spectrum of lead diiodide vapour was measured at 775 K and values were assigned to the fundamental vibrational frequencies. The results were used with other known molecular parameters to calculate the thermal functions of the gas. The vapour pressure over the solid was measured by the Knudsen effusion technique in the temperature range 580 to 621 K. The results were subjected to a second-law and third-law analysis and the enthalpy and entropy of sublimation were calculated.