The effect of the sulfates and alkalis on the durability of Portland cement systems was investigated through a series of cube and prism mixes. Durability was assessed using expansion of mortar prisms and the compressive strength of mortar cubes. The study covered a large range of both alkali and sulfate contents using 5 different Portland cements. The alkali contents ranged from 0.27 to 3.8%, the sulfate content (as SO3) ranged from 2.54 to 5%. Doping was done using Terra Alba gypsum and potassium hydroxide. In addition to physical measurements, SEM, XRD, chemical analysis and heat of hydration calorimetry were used for further analysis. Mixing, curing and testing were done at room temperature. The results show that sulfate contents up to the levels used in this study, at low alkali contents and ambient temperature curing, did not adversely affect durability of Portland cement mortars up to 360 days. A correlation was established between expansion and ettringite formation. Increasing the alkali content always resulted in loss of compressive strength, and in some cases, excessive expansion. Excessive expansion was only experienced at the 3.8% level. Alkali levels of up to 2% and sulfate levels of 5% did not result in excessive expansion at room temperature-cure up to the ages reported here. The effect of alkali depended on thecement mineralogical composition, especially C3S content. The addition of alkalis seems to impact the nature of the microstructure and the nature of other hydration products. The addition of sulfates seems to counteract the effect of alkalis, especially on the loss of compressive strength. However, these sulfates may result in other problems as they may be available at any time to form ettringite which may, under certain conditions, result in excessive expansion. It was concluded that sulfate levels on the order of 3-3.6%, did not pose any major durability drawbacks under normal curing temperatures and low alkali contents (<1%). Alkali levels above 1% will adversely affect the durability of Portland cement systems.