Sulfoxylate SO2H- (SO2 2-), a strong reducing agent readily produced by hydrolysis of thiourea dioxide, reacts with ferric myoglobin (Mb) to reversibly produce Fe(II)-Mb, starting from either aerobic or anaerobic conditions. Exposure of Fe(II)-Mb to excess sulfoxylate further produces Fe(II)-CO-Mb. Fe(II)-Mb can be regenerated by reoxidation with ferricyanide at this stage; hemin, rubredoxin and cytochrome c show a similar reactivity towards sulfoxylate. The source of CO is not the protein moiety, nor is it the heme or the thiourea dioxide – but rather CO2, via its reaction with sulfoxylate when the latter is used in large excess. These findings provide a convenient single-step route to carbon monoxide heme adducts, without the need to manipulate toxic CO gas. Financial support from the Romanian Ministry of Education and Research, (CEEx-ET98/2006, to RSD), Russian Foundation for Basic Research and Romanian Academy (joint grant to RSD and SVM), and “Babes-Bolyai” University (Scholarship for Scientific Research to KZ) is gratefully acknowledged.