Magnesium corrosion in HPMg, AZ91 and ZE41 was studied in vitro with Hank’s Solution as the Simulated Body Fluid (SBF). Corrosion rates in pH 6.5, 7.0, 7.4, 8.0 and 8.5 were studied using weight loss and hydrogen evolution. Microstructures were observed using Scanning Electron Microscopy (SEM). CO2 was introduced into the SBF to control pH at its proposedlimits. Actual pH values were left-shifted from the nominated pH range, was tentatively attributed to instability of experiment, longer duration taken to reach the pH limits and activities of ion concentrations under different pH conditions. Lower corrosion rates in pH 8.5 experiment was ascribed to no input of CO2 into the SBF and proposed pH limit was not attained until the end of experiment. No significant relationship was indicated by the corrosion rates towards pH change. Corrosion rates of specimens were in the sequence of HPMg < AZ91 < ZE41.