The 135Cs/137Cs isotopic ratio allows to characterize the potential sources of radioactive contamination in the environment. This ratio is usually determined in highly contaminated environmental samples by combining selective radiochemistry with mass spectrometry measurement. In contrast, few data are available at the environmental level of 137Cs (< 1 kBq.kg−1) due to the occurrence of interferents (Ba, Mo, Sb and Sn) in the mass range of interest and due to the very few radiocesium atoms. To obtain enough radiocesium from an environmental material with low levels of radioactivity, a large sample must be analyzed. In the present work, a new method of Cs extraction and purification has been developed allowing for the first time to analyze 100 g of ash of soil or sediment in only one week. The achieved chemical performances are excellent with a 137Cs recovery yield from soil and sediment samples, deduced from gamma spectrometry, above 80% and decontamination factors of 1.4 × 107, 5.7 × 104, 2.3 × 105 and 1.5 × 104 for Ba, Mo, Sb and Sn respectively.