Arsenic (As) is a major pollutant in many mine tailings resulting from base metal and gold deposits. It can potentially cause environmental risks because of its high toxicity at low concentrations. A new tailings' management technique has become popular in new mines, the cemented paste backfill (CPB). It consists of mixing tailings with water (typically 25%) and a low proportion of hydraulic binder (3-7%) to produce a paste that can be stored in underground mine openings. Even if CPB is mainly used for improving mining techniques and for tailings storage purposes, it could also provide environmental advantages by stabilizing contaminants such as As. In the present study, X-ray absorption fine structure (XAFS) investigations were conducted on CPB samples synthesized in the laboratory and spiked with As. XAFS analysis were also performed on a variety of As-bearing compounds (natural or synthetic), to provide a database of reference XAFS spectra. Among these reference samples, calcium-silicate-hydrates (C-S-H) spiked with various amounts of As through adsorption and co-precipitation were synthesized. The nature and amounts of the As species in the CPB samples, as well as As valence state were determined by linear combination of the reference spectra, in a least squares fitting procedure. As speciation in C-S-H was determined by extended X-ray absorption fine structure (EXAFS) fitting using theoretical curves from ab initio calculations. The results indicate that the binders promote the oxidation of As(III) to As(V) in the CPB. The As species formed in CPB are composed mainly of calcium arsenates, but other secondary minerals such as ferric arsenate can be present. Moreover, the use of fly ash binder could promote the formation of calcium-iron arsenates. However, CPB samples do not seem to contain arsenical C-S-H, as shown by the fitting results involving the synthetic C-S-H. In these samples. As reacts mainly through sorption mechanisms. (C) 2012 Elsevier Ltd. All rights reserved.