The sorption of americium on marl colloids is studied in a comprehensive way. In Na-HCO3 groundwater conditions, americium in the concentration range 3xE-11 – 3xE-10 M sorbs onto groundwater colloids. In these tests, the colloid concentration is rather high, i.e. Ccoll 1.7+-1.3 mg/L for sizes 50–5000nm or Ncoll (1.7+-0.1)xE11 l-1 for sizes larger than 100 nm. These colloids are mainly chlorite and smectite/illite with an average size smaller than 500 nm. For the groundwater (pH 8.6, [Na+] [HCO3-] 2xE-2 M), Kd values in the range 1–3 E6 ml g-1 are measured. These values are compared with experimental and modelled values reported earlier in the literature.