The mechanism of the reaction of [1,2-benzisotellurazol-3(2H)-one], ebtellur derivative 2_Te, with peroxynitrous acid, HOONO, was elucidated at the density functional (B3LYP) level by using triple-zeta quality basis sets (BS1). It was found that in the gas phase the reaction, proceeding via the concerted pathway, occurs with DeltaH = 0.3 (DeltaG = (3.4)) kcal/mol HO-ONO bond cleavage barrier. The DeltaG value of this barrier is only slightly larger than the (2.2) kcal/mol required for the HO-ONO bond homolysis. Inclusion of the solvent effects at the single-point PCM level increases the HO-ONO activation barrier to 5.8 kcal/mol. The product of the HO-ONO bond cleavage by 2_Te is the complex 2_Te(OH)(ONO), which could decay via two distinct pathways leading to NO2. + 2_Te(OH)(.) and 2_Te(O) + HONO. Both processes are endothermic in the gas phase: 23.7 (11.4) and 26.4 (14.5) kcal/mol, respectively. The inclusion of solvent effects makes the 2_Te(O) + HONO formation more favorable in water. The comparison of the reaction of 2_M + HOONO for M = Se and Te shows that the HO-ONO bond cleavage is fast for both metals, in the gas phase. However, the probability of competition in the concerted and stepwise pathways is higher for M = Te than for M = Se. The reaction, of 2_Se with HOONO produces only selenoxide 2_Se(O), while the reaction of 2_Te with HOONO may produce the intermediate 2_Te(OH)(ONO) and 2_Te(O) + HONO. We also compared the mechanisms of the reactions of 2_Te with ONOO- and HOONO.