A number of silica supported nickel carbonate systems were studied by positron annihilation lifetime spectroscopy (PAS) and nitrogen gas adsorption method (BET) to evaluate the pore size distribution in the samples. Results show that the long-lived o-Ps component gives a pore size much smaller than the average pore radius obtained from BET analysis. This indicates that o-Ps represents the pores of smaller size in a sample with wide pore size distribution.