The photocatalytic efficiencies of TiO2 nanocrystals of different modifications (anatase, rutile, P25 Degussa, Hombikat), to oxidize cyanide ion and subsequently the cyanate also, under natural sunlight at 950±25W m.2 in alkaline solution have been compared. The oxides have been characterized by powder XRD, UV-visible diffuse reflectance and impedance spectroscopies. Under identical solar irradiance, the reaction follows Langmuir-Hinshelwood kinetics on cyanide, and depends on the apparent area of the catalyst bed and dissolved oxygen. However, the adsorption of cyanide on TiO2 in dark is too small to be measured analytically. The photocatalytic activity of TiO2 is not solely governed by the band gap or charge-transfer resistance or capacitance or phase composition but is in accordance with the specific surface area or the average crystallite size; rutile is an exception.