Intradialytic hypotension remains the most common complication of routine outpatient hemodialysis treatments. There is debate as to the optimum dialysate sodium concentration, with hypotonic dialysates potentially causing intracellular swelling and hypertonic ones intracellular dehydration. Multi-frequency bioimpedance was used to assess extracellular and intracellular fluid volumes in 53 adult hemodialysis patients. Dialysate sodium was checked by ion electrophoresis. The mean decrease in extracellular volume and intracellular volumes were 1.01 ±0.09 and 0.88 ±0.18 kg, respectively. The median dialysate to sodium gradient was -3 mmol/L (-1 to -6), with a median dialysate sodium of 136 mmol/L (136-138). There was no association between changes in body fluid composition and sodium concentrations, or gradients. The mean difference between dialysate sodium prescribed and delivered was 2.4 ±0.8 mmol/L. In this study we were unable to demonstrate a relationship between predialysis serum sodium and the dialysate sodium prescribed and changes in extracellular or intracellular fluid volumes. However this study showed that using a negative sodium gradient, patients can be successfully ultrafiltrated without setting up intracompartmental fluid gradients. The caveat is that the prescribed dialysate to serum sodium gradient may differ from the actual gradient.