AbstractAmong diatoms, Phaeodactylum tricornutum is a peculiar species that exists in three morphotypes with distinct cell wall structures and low silica content. X‐ray photoelectron spectroscopy (XPS) analysis was performed on P. tricornutum and compared with diatom Thalassiosira pseudonana; the results provide new information on the chemical composition (elements, chemical functions, classes of biochemical compounds) of the cell surface. Two types of silicon were found: condensed silica (SiO2) and weakly polymerised silicate. Cells of T. pseudonana showed the highest concentration of silicon, with a majority in the form of condensed silica. For the fusiform and triradiate morphotypes of P. tricornutum, the majority of the small concentration of silica found was in the form of weakly polymerised silicate. For all morphotypes of P. tricornutum, higher polysaccharide concentrations replaced silica as a structural part of the cell wall. In both diatoms, a high concentration of lipids was measured, in the form of carboxylic esters. Protonated nitrogen and phosphate were found in correlated amounts and attributed not only to phospholipids but also to phosphoproteins. Chloride ions characterised by a high electron density might be associated to these moieties. Sulfate groups were also detected, principally in P. tricornutum, and attributed to monoesters of polysaccharides.