When mutual positive correlations occur between δ18O, 87Sr/86Sr initial ratio (Ro), and SiO2 in continental flood basalt suites, they are generally accepted as being due to crustal contamination. In continental flood basalt suites that have undergone coupled assimilation fractional crystallization in which the contaminant is granitic continental crust, Ro reflects the degree of contamination and will correlate positively with SiO2, which reflects the degree of differentiation. Posteruptive alteration of a suite having a range of SiO2 values should result in a positive correlation between SiO2 and δ18O because the ability of a volcanic rock to concentrate 18O depends, in part, on silica content. Suites that have undergone assimilation-fractional crystallization followed by alteration after eruption may therefore exhibit a positive correlation between δ18O, SiO2, and Ro. The δ18O data in such suites may consequently suggest erroneously high degrees of contamination.