The influence of sodium chloride (NaCl) and dibasic sodium phosphate (Na2HPO4 center dot 2H(2)O, DSP) on the phase transformation and particle size of titanium dioxide powder had been investigated. The salt matrix was shown to suppress both the amorphous to anatase transition and the anatase to rutile transition. DSP was particularly effective. Irrespective of the additives, transformation versus time curves of the anatase to rutile conversion were observed to be sigmoidal, and were interpreted in terms of a first order, nucleation-growth controlled phenomenon. Analysis of these curves using appropriate rate laws yielded activation energies for nucleation of similar to 4.3 eV for NaCl salt matrix and similar to 8.6 eV for DSP salt matrix, compared with similar to 3.2 eV for a sample with no salt. Activation energies for the growth or propagation stage were similar to 5 and similar to 8.9 eV for NaCl and DSP powders, respectively, compared with similar to 3.8 eV for no salt. In the solid state, salt matrices suppressed the particle growth as well. These behaviors, in general are thought to be governed by the presence of salt impurities, especially anions chemisorbed at the surface.