A systematic electrochemical study was performed to understand the influence of HEPES buffer concentration on the biodegradation behaviour of pure magnesium in a pseudo-physiological solution (Earle's balanced salt solution (EBSS)). Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation experiments suggest that HEPES accelerates the degradation of magnesium. While 5% CO2 in EBSS reduced the polarisation resistance (R-p) of magnesium by similar to 79%, addition of HEPES (25 mM) to EBSS decreased the R-p of magnesium by similar to 98% and escalated the corrosion current (i(corr)) by over an order of magnitude as compared to that in EBSS. Increase in HEPES concentration (50 mM and 100 mM) further increased the degradation of magnesium. Interestingly, the bulk pH difference between EBSS + HEPES and EBSS + CO2, before and after the electrochemical testing, was only marginal. However, the polarisation curves and Fourier transform infrared (FTIR) spectroscopy analysis suggest that HEPES affected the formation of insoluble salt layer (phosphate and carbonate) on the surface of magnesium. This effect can be attributed to the increase in the ionic strength of the solution due to HEPES addition. (C) 2017 Elsevier B.V. All rights reserved.