Ascorbic acid and ascorbate ion (denoted together as ASC) inhibit nitrosation by competing for the nitrosating agents formed from nitrite (e.g. N2O3 NO+ and NOSCN). ASC is oxidized irreversibly by this reaction and the nitrite equivalents arereduced to nitric oxide (NO). In open, aerobic systems the effective stoichiometry of the reaction between ASC and nitrite is not fixed, but is determined by a competition between the physical removal of NO (and NO2) from the system and the oxidation of NO by dissolved 02. The oxidation of NO reconverts it to a nitrosating agent which may react again with the remaining ASC. To determine the conditions under which ASC is most effective as a nitrosation inibitor, we examined the kinetics of the reactions between nitrite and ASC and between nitrite and proline. These reactions were studied in open shaker flasks as functions of pH, anion composition (SCN− and Cl−), temperature, and gas-liquid masstransfer rate. At lower mass transfer rates, at lower pH and/or in the presence of SCN− or cl−, relatively more ASC was consumed by a given amount of nitrite. Increased temperature caused more or less ASC to be consumed by a given amount of nitrite,depending on the conditions. A mathematical model of the reactions and mass transfer steps was developed which describes each of these features. The model predicts the variable stoichiometry of the reaction between nitrite and ASC in open, aerobic systems, and clarifies the mechanisms by which ASC inhibits nitrosation.