A selective catalyst for the CO methanation should show a temperature range in which the complete CO conversion occurredwithout or with an acceptable low level of CO2 methanation. Then,a comparison between CO conversion and the R ratio (R = moleof formed CH4/mole of reacted CO) was carried out. R equal to1 simultaneously with a complete CO conversion indicates thatthe produced CH4 derives totally from the reaction of CO with H2(methanation selectivity toward CO equal to 1), whereas R > 1 witha complete CO conversion is an index of also a certain degree of CO2methanation