Terra Sigillata is a fine ware produced during the Roman period. Its great success was because of its remarkable gloss (or slip), which provides a bright deep red color. Transmission electron microscopy and X-ray powder diffraction techniques were used to characterize the microstructure and microcom position of Roman period Terra Sigillata slip from two sites in southern France. Investigations on eight samples are presented. The slip coatings have similar structures and compositions, consisting of sub-micrometric hematite (<=mu m) and nanometric corundum (few tens nm) crystals in a glass matrix with inclusions of spinel and quartz crystals of varying size depending on the extent of preparation by eleutriation. The firing temperature, determined in previous studies, was in the range of 1020 degrees-1080 degrees C. The matrix does not contain iron or other metallic ions. Hematite and corundum crystals are homogeneously dispersed in the matrix and, together, confer the specific red-orange color to the sigillata.