Atomic structure of the Sr-adsorbed Si(100)(1 × 2) surface has been investigated by scanning tunneling microscopy (STM) and ab initio calculations. This surface reveals rows of Sr atoms between unbuckled Si dimer rows as well as an abundance of vacancy defects in the metal rows. The density of such defects can be minimized by the optimization of growth procedure; however, they cannot be avoided completely, forming vacancy lines along the [021] directions, where the neighboring vacancies are connected via the Si dimer. The origin of vacancy defects is discussed in the context of Sr/Si(100)(1 × 2) and related surfaces. In addition, the interaction of Sr/Si(100)(1 × 2) with oxygen is examined by STM directly during the exposure in the O₂ gas.