A comparison of the mild hydrocracking activities and selectivities to ring opening of Pt supported on pillared. and delaminated clays is presented in this study. The catalysts were prepared as extrudates with 30% alumina and were used to. process a gas oil fraction derived from Canadian oil sands that was first hydrotreated to reduce the heteroatom content. Experiments were performed in both hydrogenation and mild hydrocracking regimes using a fixed-bed microreactor between 260, and 380 degreesC, at a total pressure of 10.3 MPa and a LHSV of 1.0. Feed and total liquid products were characterized by using ASTM standard methods and by GC-MS for, compositional analysis. Results show that under similar reaction conditions, Pt-supported pillared clay had higher hydrogenation and hydrocracking activities than Pt-supported delaminated. clay. However, the latter catalyst had a higher selectivity to diesel yield and produced less naphtha for the same conversion of the +343 degreesC fraction. Detailed compositional analysis of total liquid products over the operating temperature range also indicated that the Pt/pillared clay catalyst achieved ring opening of two- and three-ring naphthenes, while the Pt/ delaminated, clay converted only three-ring species. Catalyst deactivation - was evaluated by monitoring feedstock conversion and by examination of fresh and used catalysts after approximately 300 h time-on-stream. The surface area and pore size distributions measured by N-2-adsorption/desorption of fresh and used catalysts were compared to determine loss of microporosity due to deposition of carbonaceous materials and possible changes to the catalyst structure due to the reaction environment. Results suggest that pores with diameters less than 60 Angstrom were more susceptible to coke formation.