The determination of the proportion between the biogenic and the fossil-derived fraction in carbon dioxide emissions from industrial stationary sources is a relevant aspect in the frame of the worldwide efforts to reduce greenhouse gas emissions into the atmosphere. In this field the 14C-based method has gained importance over the years and has been included in standard protocols accepted at both the national and international levels. The advantages of the method, based on the large difference in terms of 14C signature between fossil and biogenic carbon, are reviewed as well as some critical aspects related to its application. In particular the results of a study aimed at validating the method by analyzing synthetic gas mixtures produced in a dedicated plant and with a tuneable proportion between fossil and biogenic carbon dioxide are presented.