To examine the structures and growth modes of adsorbed layers on heterogeneous surfaces, we performed simultaneous measurements of adsorption isotherms and X-ray diffraction patterns of Kr, N-2, and H2O on the poorly hydroxylated surface of pyrogenic silica at 80, 50, and 270 K, respectively. The adsorption of N-2 resulted in a type II isotherm, whereas that of H2O led to a type III isotherm. The adsorption isotherm of Kr was intermediate between them. The analysis of the diffraction patterns showed that the molecules of Kr and N-2 adsorbed on pyrogenic silica formed simple triangular lattices of medium-range order, and thus, they were nearly close packed on the heterogeneous surface. On the other hand, the H2O monolayer did not possess even short-range order. At a monolayer capacity of the Brunauer-Emmett-Teller (BET) equation, the nitrogen cross-sectional area was 15.5 Angstrom (2); the difference with the frequently used value of 16.2 Angstrom (2) was only 4%. At the BET monolayer capacity, neither Kr nor H2O molecules covered completely the surface of the hydroxylated silica.