The precise nature of the reaction between acids (HCl and CH3COOH) and alkali ortho-arsenite has been investigated by means of pH, E.M.F. and conductance measurements. The well-defined breaks and inflexions in titration curves provide cogent evidence for the formation of meta-arsenite, NaAsO2, and meta arsenious acid, HAsO2, at pH ranges 9.7-10.2 and 4.5-5.5 respectively; the formation of pyro-arsenite Na4As2O3, is, however, not indicated. The shape of the titration curves reveals that ortho-arsenite does not exist as such but hydrolyses to form a mixture of NaOH and lower arsenites.