One of the most important tasks of nuclear safeguards is to control the activities in nuclear facilities worldwide. These activities include the enrichment of 235 U: To identify enrichments above the maximum authorised level in commercial enrichment plants (usually 5% 235 U) swipe tests are performed and the 235 U enrichment is determined in the sampled uranium oxide particles. A method is described that allows to measure the 235 U content in small particles (around 1 mm in diameter) by the use of fission tracks. It is based on the fact that in uranium fuel, only 235 U atoms are fissile with thermal neutrons and contribute to the amount of fission tracks counted for individual particles. Standard uranium oxide particles with a defined diameter and isotopic composition were prepared and used to calibrate the method. Results show that the detection efficiency has a non-linear dependence on the number of fission tracks per particle. The enrichment in 235 U for single particles can be determined when good size measurements of the particles in a sample are available.