Separate fractions of dentine and enamel of 12 individuals from the necropolis of Sion (Valais, Switzerland) have been analyzed for Pb and Sr isotope compositions. The individuals span a time period of more than 1 ka from ∼5.2 ka B.P. to ∼4.1 ka B.P. and include Middle Neolithic, Upper Neolithic, Bell Beaker and Old Bronze age civilizations. Individuals were buried either into or close to dolmens or into soil at 4 different sites on the alluvial fan of the Sionne stream, a tributary of the Rhone river. Isotopic composition of dentine and enamel pairs of the individuals indicate that teeth underwent post mortem contamination. Lead contamination is due to two different sources, petrol and natural Pb from soils. The preponderance of either petrol or natural Pb contamination in teeth depends on the burial sites. Differences in the content of Pb-retaining particles (organic matter, Fe-hydroxides, clay minerals), that depend largely on soil grain size, are suggested to be responsible for the differential Pb contamination at the investigated burial sites. Strontium contamination is stronger in individuals buried into or close to dolmens, which are made of calc-schist slabs. Despite post mortem contamination, both Pb and Sr isotopes have allowed the identification of an individual's enamel with isotopic compositions significantly different from those of the local substratum, suggesting his immigration to the Sion area from a geologically different region. This individual belongs to the Bell Beaker civilization, which is considered to have immigrated into Western Europe and the Mediterranean basin at the end of the Neolithic. Systematic differences between enamel and dentine isotopic compositions of all other individuals analyzed suggest that they assimilated in vivo Pb and Sr from an area nearby to that in which they were buried.