Background: Identifying the sources of variation in QTc measurements is important for preventing arrhythmias during and after hemodialysis. The present study was designed to determine the correlation between the type of hemodialysis buffer and the changes in QTc interval in patients on chronic hemodialysis. Methods: Fifty-nine patients on chronic hemodialysis who referred in winter 2007 to hemodialysis centers of Ghaem and Hashemi Nejad hospitals, in Mashhad, Iran, were divided into two groups according to their last dialysate buffer: acetate or bicarbonate. Electrocardiography, arterial blood gas parameters, serum K+, Na+, ionized calcium, and albumin levels were measured prior to and after hemodialysis in all patients. Results: All arterial blood gas parameters and serum electrolytes concentrations were increased except K+ levels that were significantly decreased with hemodialysis. PCO2 and QTc intervals were slightly increased in all patients, however this increase was not statistically significant. We found that the type of dialysate affected the QTc interval, HCO3, base excess, base excess of extra cellular fluid, and base buffer changes with no effect on ionized calcium, pH, PCO2, and serum albumin concentration. QTc interval was prolonged by using bicarbonate and shortened by using acetate dialysate buffer. We found no correlation between the variations of QTc interval and serum electrolytes or arterial blood gas parameters in either group. Conclusion: Bicarbonate buffer use in hemodialysis prolonged QTc interval and acetate buffer shortened it. This effect is independent of serum electrolytes and pH changes during hemodialysis. The effect of bicarbonate buffer is probably due to more tolerability of ultra filtration, more effective edema reduction and augmented body electro-conductivity.