The effects of chloride precursors on the Pd valence states and surface structures of Pd-M$+/ SiO, catalysts for carbon monoxide hydrogenation have been studied by means of electron-spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) techniques. The Cl-containing catalyst exhibits a much more positive Pd valency and a much more stable Pd surface structure than the Cl-free one. This is attributed to the stronger ability of chlorine anions to stabilize Pd structures and to transfer electrons from Pd to Mg'+, rather than to a better Pd dispersion of the Cl-containing catalyst. Over the Cl-containing catalyst, methanol formation and a Pd+ ESR signal are found to increase concurrently with reaction time. Accordingly, Pd+ ions are claimed to be essential for methanol formation. However, only those Pd+ ions which are located in a suitable structure are supposed to be the active center for methanol synthesis, because oxygen or hydrogen water pretreatments are found to cause (i) a marked decrease of the turnover frequencies (TOF) for methanol formation, (ii) a probable transformation of Pd(C1, X)Mg+ (X=Cl or 0) bridge structures to oxychloropalladium and MgO, and (iii) the formation of more Pd+ ions. Furthermore, the Pd+ ions formed during the induction period are found not to be mainly due to the catalyst being modified by water or methanol, but probably due both to the high-pressure Pd reconstruction and the oxidation of an HCOO intermediate. In addition, the metal-promotersupport interactions are discussed.