Arsenic and other heavy metal concentrations in post mortem soft tissues from a Portuguese King of the nineteenth century were studied by energy-dispersive x-ray fluorescence analysis. This work is an attempt to clarify the strange death of the King, through the analysis of his post mortem soft tissue remains, which were kept inside a Chinese porcelain container since his death in 1826. The container was put in a wooden coffin and buried underground in a chapel of the St. Vincente de For a Monastery in Lisbon. Quantitative elemental analysis of the remains showed very high concentrations of arsenic and lead, which on average correspond to a few hundred times the mean normal values for contemporary human soft tissues. Increased levels of Cu were found, and also small enrichments for Fe and Zn, probably due to either enrichment during the 175 years or contamination by direct intake from the container, or both. The results obtained are consistent with acute arsenic poisoning, reinforcing the conclusion from the medical anatomical-pathological report, based on the analysis of the fragments by scanning electron microscopy.