<i>Background:</i> Failure to achieve isonatric haemodialysis (HD) drives an expansion of extracellular volume leading to increased interdialytic weight gain (IDWG). This may be a causative factor in the development of HD-induced cardiac injury. We examined total and diffusive sodium mass balance during HD. <i>Methods:</i> 24 chronic HD patients using a fixed 140 mmol/l sodium concentration were studied over 4 weeks. Dialysate and plasma conductivity and ionic mass balance (IMB) were recorded. IMB estimates total ionic transfer across the HD membrane. <i>Results:</i> Mean total IMB was 338 mmol indicating net sodium removal. Intrapatient variability was less than interpatient variability (coefficient of variation = 42 vs. 26%, respectively). The diffusive component of ionic mass balance (IMB_diff) was 97 ± 18 mmol approximating 29% (±22–36) of total sodium removal. IMB_diff also correlated with both plasma conductivity and predialysis plasma sodium (r² = 0.82 and 0.6, respectively; p < 0.0001) as well as the reduction in plasma conductivity and plasma sodium during HD (r² = 0.7 and 0.5, respectively; p < 0.0001). <i>Conclusion:</i> HD against a fixed dialysate sodium concentration of 140 mmol/l results in a wide range of sodium removal with a mean of 29% removed by diffusion. Online conductivity monitoring can be utilized as part of a variety of strategies to enable the delivery of individualised and isonatric HD. Further study is required to explore the utility of such strategies which may be crucial in reducing IDWG and HD-induced cardiac injury.