Three soil profiles were selected from the Ejin Oasis in the lower reaches of the Heihe River in northwest China to collect soil samples at different depths and groundwater samples for determination of chemical compositions of the samples, and to measure the changes in soil moisture potentials at different depths of the profiles. The results show that SO 2- 4 is the major anion in both the soluble ions of the soil in the unsaturated zone and the ions in groundwater in the saturated zone. Both the soil and the groundwater are in a weakly acidic environment. The concentrations of soluble ions of the soil tend to decrease with depth. Both convergent and divergent zero flux planes were observed in the unsaturated zone in the oasis. From summer to winter the zero flux planes became indistinct and even vanished. The concentrations of soluble salts, water contents and total soil moisture potentials are found to have a similar tendency to change with depth. Water extraction by root systems is believed to have an important bearing on the vertical occurrence of the water and salts in the unsaturated zone in the oasis.