U-233 (T-1/2 = 0.159 My) has aroused much interest in the last few years because of the potential of U-233/U-236 to inform about anthropogenic U sources. The reported U-233/U-238 atom ratios in general environmental samples are in the 10(-12)-10(-6) range. We demonstrate that U-233/U-238 abundance sensitivities at the 5 x 10(-11) level can be achieved at the 1 MV Accelerator Mass Spectrometry system at the Centro Nacional de Aceleradores (CNA, Seville, Spain). Unresolved U-234, U-235 and Th-232 molecular fragments are identified as the main U-233 background contributors and procedures to subtract them are proposed. U-233/U-238 and U-236/U-238 atom ratios were analysed in the IAEA reference materials coded Soil-6, 300, 385, 410, and 412 at the CNA and at the 600 kV ETH AMS system for intercomparison. U-233/U-238 atom ratios for samples 412 and 385 were only reported by the ETH.