A method of determining an age in years by measuring the concentration of carbon-14 remaining in an organic material, usually formerly living matter, but also water, bicarbonate, etc. The method is based on the assumption that assimilation of carbon-14 ceased abruptly on the death of an organism and that it thereafter remained a closed system. The method is useful in determining ages in the range of 500 to 30,000 years or 40,000 years, although it may be extended to 70,000 years by using special techniques involving controlled enrichment of the sample in carbon-14.