The influence of pH on the corrosion behavior of Mg-based AZ91D alloy was investigated in a constant composition phosphate medium using various electrochemical techniques, complemented with surface analysis data. The studied solutions were 0.1 M H3PO4, NaH2PO4, Na2HPO4 and Na3PO4 having pH values of 1.8, 4.5, 9.1 and 11.8, respectively. Spontaneous passivation was substantiated from monitoring the continuous positive shift of the open circuit corrosion potential with both immersion time and solution pH. The impedance data indicated more improvement in the insulating properties of the corrosion products formed on the alloy surface with increase in pH. The electrolyte pH plays a determinant influence on surface film properties, as films formed in phosphate solutions with higher pH values are thicker, thus affording better protection for the alloy than those formed in acidic solutions. Good agreement was observed between the results obtained from electrochemical techniques and those from EDX and XRD examinations. The alloy is more susceptible to corrosion in acidic phosphate solutions than in the alkaline ones. Crystalline magnesium (Mg), magnesium hydride (MgH2) and magnesium oxide (MgO) were found to be the main constituents of the surface film after holding for 2 h in the acidic phosphate medium.