A method for passivating low-alloy steel surfaces in apparatus operating in the temperature range 350 to 580° C. and exposed to a carbon monoxide containing gas mixture comprises adding a passivating compound containing at least one phosphorus (P) atom to the gas mixture. The gas mixture is preferably a reformed gas and also described is a process for producing a synthesis gas wherein, prior to cooling a reformed gas mixture to a temperature between 350 and 580° C. in apparatus having low-alloy steel surfaces downstream of one or more reforming steps, a passivating compound containing at least one phosphorus (P) atom is combined with the gas mixture.