The radionuclides 135Cs and 137Cs are present in the environment with an isotopic ratio 135,137Cs/Cs ranging below 10-10. The isotopic ratio 135Cs/137Cs can be used for source assessment of anthropogenic cesium input into the environment and finds applications in geology, nuclear forensics and oceanography. The combination of low concentration, low beta-decay energy and long half-life prevents the determination of 135Cs via radiometric methods with the demanded sensitivity. Therefore, we established a measurement procedure for cesium by Accelerator Mass Spectrometry at VERA. First results on isobar suppression, abundance sensitivity and 135Cs/137Cs ratios of environmental samples will be presented.