[en] We have investigated the geometric structure of the Ca/Si(1 1 1)-(2x1) surface using low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. A clear (2x1) periodicity was observed in LEED after annealing the (7x1) phase formed at room temperature in LEED. In the Si 2p core-level spectra, three surface components were observed. By considering the energy shift and intensity of each surface component and the Ca 3p core-levels of the two phases, we regard the (2x1) phase to be formed by π-bonded Seiwatz Si chains with a Ca coverage of 0.5 ML