The abundance ratios of 234U, 235U, 236U and 238U were determined by mass spectrometry on uranium samples obtained from a chromatographic uranium enrichment process based on the U(IV) - U(VI) chemical exchange reaction. According to standard theory, the elementary isotope effect in processes based on equilibrium chemical exchange is proportional to the mass difference of the isotopes. In the present work this has been confirmed for 234U, 236U and 238U. However, according to the observed data the fractionation between 234U and 235U is much smaller than the one between 235U and 236U in spite of the same mass difference in either isotopic pair. The anomaly of the fractionation is concluded to be due to the characteristics of 235U, which is the only odd mass number isotope in the present system.