The depositional environment and maturity of source rocks in the southern Gulf of Suez were evaluated using biomarker and isotope data from crude oils derived from a variety of source rock types of different geological ages. Two oils families were identified and are referred to as types A and B. Type A oils are characterized by a predominance of oleanane and relatively low gammacerane concentrations, suggesting that they were derived from a terrigenous source rock with a significant input of angiosperm material inferred to occur within the marginally-mature syn-rift Lower Miocene Rudeis Shale. By contrast, type B oils are distinguished by a predominance of gammacerane and relatively low oleanane concentrations, suggesting that they were generated from mature marine carbonate source rocks inferred to occur within the Upper Cretaceous Brown Limestone and Middle Eocene Thebes Formation. Maturity parameters including the sterane isomerisation ratios C(29)alpha beta beta/(alpha beta beta+alpha alpha alpha), C(29)alpha alpha alpha 20S/(S+R) and TAS/(TAS+MAS), together with aromatic sulphur compound ratios (4-MDBT/I-MDBT, 4,6-/1,4-DMDBT, 2,4-/1,4-DMDBT, and DBT/phenanthrenes), support the higher thermal maturity of type B oils relative to type A oils. The biomarker variablility reflects the occurrence of two distinct source rocks in the southern Gulf of Suez and suggests that two independent petroleum systems are present here. These appear to be confined to the pre-rift (pre-Miocene) and syn-rift megasequences respectively.