Two preparation methods used for determining the delta 18 O of quartz overgrowths are compared from the sandstones of the Middle Jurassic Brent Group, Northern North Sea. Method one uses dilute HF to preferentially attack interfacial materials present between the overgrowth and the detrital core, with subsequent ultrasonic agitation used to break the overgrowths off at the weakened contacts. Size fractionation by sieving should concentrate overgrowths into the smaller grain size separates. In the Brent sands studied, however, little interfacial material is present between overgrowth and core, and the overgrowths are thinly developed (average width 20-50 microns). Concentration of the diagenetic overgrowths is therefore difficult. This difficulty is compounded by large amounts of detrital quartz in the size fractions, as a result of internal fracturing of the grains. The delta 18 O (SMOW) values for the finest size fraction (supposedly overgrowth rich), ranging from 9 to 13.3 per thousand , are unusually low for diagenetic quartz overgrowths, and result from the high detrital component. The second method derives the delta 18 O values for the overgrowths by the difference in delta 18 O for the quartz grains plus overgrowths and quartz grains from which the overgrowths have been removed by HF dissolution. The mass balance used requires the overgrowth abundance to be known from point counting. The calculated delta 18 O values obtained are typical for authigenic quartz overgrowths and range from 17.1 to 20.9 per thousand . Direct isolation of quartz overgrowths for delta 18 O measurement is suitable where interfacial material exists between the detrital grain and the overgrowth and where overgrowths are well developed. Otherwise, preferential HF dissolution of the overgrowths and a delta 18 O mass balance calculation is the preferred method.