A method for reducing the interaction between carbon monoxide present in a heat exchange medium and metals on the shell side of heat exchange reformer apparatus used for reforming of hydrocarbons by treatment of the shell-side of said apparatus with an effective amount of at least one passivation compound containing at least one element selected from phosphorus, tin, antimony, arsenic, lead, bismuth, copper, germanium, silver or gold is described. Where volatile compounds are formed by the passivation compound, the method further comprises compound recovery to prevent contamination and deactivation of subsequent process steps. The method reduces side reactions, provides improved reformer tube lifetime and allows tubes to be treated in-situ without the need for process shutdown.