The influence of sodium dodecylbenzene sulphonate (DOBS) and cetyldimethylbenzylammonium chloride (CBC) on the crystal growth of sodium triphosphate hexahydrate (Na 5P 3O 10) from aqueous solution was studied. The crystals formed are acicular twins with (001) as the twinning plane. The growth rate of the (0 1 ¯ 0) face and the (100) face was measured with a polarizing microscope, that of the (001) face with the rotary compensator developed by Ehringhaus. The R(σ) curves were determined in a supersaturation range of σ = 0.05 to 0.80. The growth of the (0 1 ¯ 0) face is completely stopped at a DOBS concentration above 25 mg/l, whereas it is only slightly hindered in the presence of CBC. The growth rate of the (100) face is greatly reduced by CBC but increased by DOBS. Both DOBS and CBC retard the growth rate of the (001) face. The growth of the Na 5P 3O 10 crystals is not controlled by transport in the solution and is not induced by screw dislocations. At supersaturations above 0.40 the R(σ) curve obeys the equation for two-dimensional nucleation.