Gas phase ethanol decomposition over acid carbon catalysts has been studied. The acid carbon catalysts were obtained by chemical activation of olive stone with phosphoric acid, without needing additional oxidative treatments. Different impregnation ratios, between 0.5 and 2, and activation temperatures, in the interval 400-800 degrees C, were used for the preparation of the carbons. Impregnation ratio shows more influence in the porous structure development than that observed for the activation temperature. XPS analyses reveal that the amount of stable phosphorus remaining over the carbon surface after the washing process increases with both activation temperature and impregnation ratio. FTIR spectra confirm the presence of surface phosphorus in form of phosphate and polyphosphate groups. XPS analyses suggest the existence of C-O-PO3 and C-PO3 surface groups that present acid character and of C3PO and C3P type. The increase of the activation temperature results in an increase in the proportion of C3PO and C3P surface groups. The catalytic decomposition of ethanol over the activated carbons yields mainly dehydration products, mostly ethylene with lower amounts of diethyl ether. In absence of oxygen the catalysts suffer a progressive deactivation. However, the presence of oxygen produces a significant increase of the ethanol conversion without any significant change in the selectivity of reaction and avoids deactivation of the catalysts under the operation conditions studied. The results suggest that oxygen inhibits coke deposition on the acid active sites. Ethanol conversion remains practically constant and selectivity does not change when water vapor is added to the reactor feed in concentration similar to that of bio-ethanol. (c) 2011 Elsevier B.V. All rights reserved.