Abstract: Authigenic kaolinite and illite are important diagenetic minerals in the Magnus Sandstone, a giant oil reservoir in the northern North Sea. These clay minerals, separated from three wells, show considerable ranges in their oxygen isotopic composition (δ18OSMOW = +9 to +16‰) and hydrogen isotopic composition (δDSMOW = -55 to -105‰). The variations in δ18O and δD are positively linearly correlated with a high degree of statistical significance for both kaolinite and illite: Kaolinite: n = 12; δD = 6.1 δ18O − 169; r = 0.66(>95%) Illite: n = 11; δD = 5.9 δ18O − 159; r = 0.78(>99%). Formation of the clays in a pore fluid of uniform isotopic composition over a range of temperatures appears unlikely. It is suggested that the observed relationships between clay mineral δ18O and δD are perhaps best explained by a model of precipitation at more or less constant temperature from pore fluids which varied isotopically across the oilfield. The isotopic composition of the formation waters would then lie along the line: δDw = 6.2 δ18Ow − 50. This is most plausibly interpreted as a mixing line with suggested minimal endmembers at (δ18O, δD) values of (+4, −24) and (−4, −76). The first of these represents reasonable isotopic values for Magnus Sandstone formation waters. Although δ18O of the second is compatible with an evolved Cretaceous meteoric water, its δD value is difficult to understand in the context of the model.