Early diagenetic carbonate cements are a common feature of Quaternary alluvial conglomerates in Oman. Cements are formed in the vadose and, more commonly, phreatic zones from near-surface groundwaters. In drainage areas underlain by the Semail Ophiolite, groundwaters have Mg (super 2+) /Ca (super 2+) ratios greater than two, and cements are often dolomite or high-magnesium calcite in addition to low-magnesium calcite. In drainage areas underlain by limestone, groundwaters have Mg (super 2+) /Ca (super 2+) ratios of around one or less and cement mineralogy is nearly always low-magnesium calcite. The oxygen and carbon stable isotopic ratios of the cements vary widely, from -10.6 per thousand to + 3.0 per thousand PDB and from -10.0 per thousand to + 0.7 per thousand PDB, respectively. Cement delta 18 O values principally reflect variation in rainfall delta 18 over a time scale of several thousand years. Rainfall and cement delta 18 O values probably are inversely correlated with the mount of rainfall, which is related to the frequency and intensity of the Indian Ocean monsoon. Thus, cement delta 18 O is potentially a proxy indicator of relative rainfall and monsoon activity. For each of three sampling areas, delta 13 C is positively correlated to delta 18 O. Cement delta 13 C values are also related to rainfall amount because rainfall controls the plant population. Greater plant respiration of isotopically depleted CO 2 to shallow groundwaters and burial of organic material in conglomerate deposits results in lower cement delta 13 C values compared to periods of lesser plant activity.