Head-Space Gas Chromatography-Mass Spectrometry (HS-GC/MS) was used to determine blood carboxyhemoglobin (COHb%), especially in cases involving blood decomposition or death, when blood extraction is impossible. Until now, the problem eluded complete solution using forensic toxicology. Therefore a method was developed using Headspace Gas Chromatography-Mass Spectrometry (HSGC/MS), which should enable the determination of COHb in decomposed blood or hepatic tissues. The technique has a clear advantage over other methods such as UV spectrophotometry. COHb% in hepatic samples were determined, which were stored at different temperatures (-20°C for 1-2 years, 0°C, and 18°C for two months). Using a column packed with molecular sieve, CO levels were quantified down to 0.01% in the air and COHb levels down to 0.2% in small blood samples quickly. The 3-min procedure requires only 0.25ml of blood sample or 1g of hepatic tissue, each time. It was shown that the method has good reproducibility with RSD of the COHb <1%. The method was accurate and reliable to investigate deaths related to CO exposure, by determining COHb and CO levels.