Carbon and oxygen isotopic compositions have been determined for Holocene reefal carbonate grains and cements in samples from Enewetak, Bikini, Bermuda, and Belize to identify components that exhibit systematic isotopic fractionation relationships. These findings are compared with fractionation relationships determined from experimentally precipitated low-magnesium calcite (LMC), aragonite, and high-magnesium calcite (HMC) to determine if these relationships are observed in naturally precipitated carbonates. Compositional variation within individual samples is extreme (up to 6‰ in δ13C PDB and 3‰ in δ18O PDB). Experimental fractionation relationships fail to predict the range of observed compositions. Aragonite cements are enriched 1.0‰ to 1.4‰ in 13C and 1.5‰ to 2.0‰ in 18O relative to calcite equilibrium compositions; HMC cements are enriched by 0.3‰ to 0.4‰ in 13C and by 1.8‰ to 1.9‰ to 18O. For carbon, these results are in agreement with the experimental work of Rubinson and Clayton. For oxygen, our results suggest that fractionation factors of both aragonite and HMC may have been underestimated. Skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements, suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co-occurring skeletal carbonates.