The reaction kinetics for low concentrations of sulfite close to the stoichiometric ration of oxygen was investigated in a rapid-mixing apparatus. Air- or pure oxygen-saturated solution containing the catalyst was mixed with aqueous sulfite solution. Mixing ratios with stoichiometric excess of dissolved oxygen were tested. Reaction orders for sulfite, oxygen, and a Co(II) catalyst from literature were confirmed in the tested concentration range. The apparent kinetic constant and activation energy data are presented. The high importance of proper mixing of the two reacting solutions is reported, which was revealed during the development of the precise experimental technique providing reliable kinetic data.