Molecular iodine dissociates in the presence of O(2)((1)Δ) and O(2)((1)Σ) molecules, but the mechanism of this process is not completely understood. In this paper, using flow tube experiments, we studied the initiation stage of iodine dissociation. Absolute spectral irradiance measurements were employed for measurements of concentrations of electronically excited particles. It was found that under the present experimental conditions initiation of iodine dissociation is mainly caused by reaction O(2)((1)Σ) + I(2) → O(2)((3)Σ) + 2I with a rate constant of (9.3 ± 2.0) × 10(-11) cm(3) s(-1). An appreciable role of the O(2)((1)Δ) molecule as the dissociation initiator has not been observed. It was observed that the growth rate of iodine atoms sharply accelerated when the concentration of I((2)P(1/2)) atoms approached 2 × 10(-4) of O(2)((1)Δ) concentration or when production rates of O(2)((1)Σ) molecules in reactions O(2)((1)Δ) + I((2)P(1/2)) and O(2)((1)Δ) + O(2)((1)Δ) became equal.