The pore size distribution was determined on brick of good and bad durability. Brick of poor durability showed a preponderance of pores smaller than one micrometer while good brick exhibited a majority of pores larger than two micrometers. Raw material composition and forming method were found to have a major influence on pore size. Grog and shale particles larger than 200 mesh favored the formation of larger pores. Pure clays produced pores smaller than 0. 1 micrometer. Soft molding produced larger pores than extrusion. Higher firing temperatures and increasing soaking times reduced the quantity of pores but not the predominant pore size in a span of normal firing temperatures. poor durability was exhibited by brick of low porosity surface and high porosity interior. -- AATA