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Molecular iodine dissociates in the presence of O2(1Δ) and O2(1Σ) molecules,but the mechanism of this process is not completely understood. Inthis paper, using flow tube experiments, we studied the initiationstage of iodine dissociation. Absolute spectral irradiance measurementswere employed for measurements of concentrations of electronicallyexcited particles. It was found that under the present experimentalconditions initiation of iodine dissociation is mainly caused byreaction O2(1Σ) + I2→O2(3Σ) + 2I with a rate constant of (9.3± 2.0) × 10–11cm3s–1. An appreciable role of the O2(1Δ) moleculeas the dissociation initiator has not been observed. It was observedthat the growth rate of iodine atoms sharply accelerated when theconcentration of I(2P1/2) atoms approached 2× 10–4of O2(1Δ)concentration or when production rates of O2(1Σ) molecules in reactions O2(1Δ)+ I(2P1/2) and O2(1Δ)+ O2(1Δ) became equal. [ABSTRACT FROM AUTHOR]