Electronically excited iodine atoms (52P½) resulting from the flash photolysis of HI have been observed using kinetic spectroscopy in the vacuum ultra-violet, and the decay of these excited atoms to the ground state (52P) has been monitored. Approximately one-fifth of the atoms observed at short delay are found to be in the excited state and result from direct photolysis via the Q0 transition. A stationary state period is observed for I(52P½) during which the rate of production of excited atoms from the reaction between H + HI is balanced by spin orbit relaxation. Outside this stationary period, collisional deactivation is predominant and relaxation can be studied in the absence of initiation processes. The contributions to the decay of I(52P½) by the constituents in the resulting HI + H2+ I2 mixture have been determined. Although the deactivation efficiency of I2 is high, this molecule has little effect on the decay rate on account of the slow rate of atomic recombination. H2 makes a small contribution to the decay. Spin orbit relaxation is determined primarily by the amount of unreacted HI present. Similar results have been found for D2 and DI. The relaxation of I(52P½) in the presence of a gas of equal mass to hydrogen, namely, helium, in which collisional deactivation would require all the energy to be converted to translation, is controlled by diffusion to the vessel wall in agreement with previous observations on argon. The deactivation cross-sections for I(52P½) with H2, D2, HI and DI are 1·6 × 10–3, 2·8 × 10–3, 1·3 × 10–2 and 1·1 × 10–2Å2, respectively. The present data on the deactivation of I(52P½) by various gases is discussed.