In a study on syngas combustion in a swirl-stabilized combustor, red deposits quickly formed on the quartz combustor walls, preventing laser diagnostic measurements. Analysis of the deposit showed that is was composed of iron and nickel. A literature review revealed that typical pressurized cylinders of CO, even at the highest CO purity level, contain up to 100 ppm(v) of iron pentacarbonyl and somewhat smaller levels of nickel tetracarbonyl. Only semiconductor grade CO stored in aluminum cylinders has sub-ppm levels of carbonyls. The demonstrated strong flame inhibition effect of carbonyls suggests that the use of CO from steel cylinders in combustion experiments may affect studies of flame ignition or extinction, in addition to the effect of particle formation and deposition observed here.