The orientation and symmetry of low-coverage ethylene adsorbed on Pd(110) at 150 K were studied by high-resolution electron energy-loss spectroscopy (HREELS) and near-edge x-ray absorption fine structure (NEXAFS) analysis. HREELS measurements unambiguously revealed that ethylene is pi-bonded with the C-C axis preferentially aligned along the [<1(1)over bar>0] row; this finding was also confirmed by NEXAFS. The molecular symmetry of this species was found to be C-1 and thus, we proposed that the C-C axis is tilted against the surface. The effect of coadsorption with hydrogen was examined, and a flat-lying species with C-2 symmetry was found to dominate on H-covered Pd(110). Furthermore, the coadsorbed hydrogen induced the C-C axis rotated about the surface normal, possibly reflecting the zigzag phase formation of adsorbed hydrogen atoms. (C) 2000 American Institute of Physics. [S0021-9606(00)70313-8].