In situ EPR and IR investigation by using CO as probe molecules show that even pre-reduced by H-2 at lower temperature results in SMSI for anatase titania supported palladium catalyst, but not for rutile titania supported palladium catalyst, which is attributed that the Ti3+ ions produced by reduction of Ti4+ are fixed in the surface lattice of TiO2, as rutile titania is more thermodynamically and structurally stable than anatase titania so that the Ti3+ ions fixed in the surface lattice of anatase TiO2 is easier to diffuse to surface of palladium particle than one in the surface lattice of rutile TiO2. Anatase titania supported palladium catalyst 0.075% Pd/TiO2 (A) pre-reduced by H-2 at lower temperature has higher selectivity of alkenes for the liquid phase selective hydrogenation of long chain alkadienes than rutile titania supported palladium catalysts 0.075% Pd/TiO2 (R). For titania (rutile or anatase) supported palladium catalysts, the elevation of pre-reduction temperature from 200 to 450 degreesC gives rise to sharp change of catalytic properties, especially for selectivity of alkenes. The very different catalytic properties between 0.075% Pd/TiO2 (R) and 0.075% Pd/TiO2 (A) catalyst pre-reduced at lower temperature, and the rapid increasing of selectivity of 0.075% Pd/TiO2 (A) and 0.075% Pd/TiO2 (R) with the elevation of pre-reduction temperature are reasonably explained by the presence of SMSI both for anatase titania supported palladium catalyst pre-reduced at lower temperature, and titania (rutile and anatase) supported palladium catalyst pre-reduced at higher temperature. (C) 2004 Elsevier B.V. All rights reserved.