There is an interesting correction (J Am Chem Soc, 2002, 124, 1553) to an earlier article on use of carbon monoxide as a Lewis acid catalyst for Friedel-Crafts reactions (Ogoshi S, J Am Chem Soc, 2001, 123, 8626). The activity of the CO was variable, and, after discussions with Prof George Olah, attributed the variability as due to a trace impurity in the CO gas. Olah had found that CO supplied in an aluminium cylinder gave low yields, whereas from a steel cylinder the yields were high! The gas lost its efficiency when passed through a silica or charcoal column, or when washed with sulphuric acid. Thus CO seems NOT to be a catalyst for Friedel-Crafts reactions, but obviously the unknown impurity in the CO is a very good catalyst at extremely low concentration.