This International Standard details analytical methods suitable for analysing the two primary toxic combustion gases, carbon monoxide (CO) and hydrogen cyanide (HCN), in blood samples collected from fire casualties. In blood, CO is measured as carboxyhaemoglobin (COHb) and HCN as cyanide ion (CN−). Although numerous methods are reported in the literature for performing blood COHb and CN− analyses, the analytical methods included herein are based upon their suitability for performing the analysis on ante-mortem and post-mortem blood samples from fire casualties. The analytical principle, analysis time, repeatability, reproducibility, robustness, effectiveness and instruments used are considered for those methods. Some of the methods described herein might not be suitable for analysing putrid or clotted blood. Burned (solid) blood can be analysed after homogenization.