Detailed analyses of oxygen isotope–water relations in <3, 13, and 109 Ma old mid-ocean ridge basalts reveal that, to a first approximation, the low-temperature alteration of deep-sea basalts in the upper few hundred metres of the igneous crust can be modelled as the partial conversion of basalt to smectite formed in isotopic equilibrium with seawater. Second-order variations in δ18O versus H2O+ plots are used to show that this approximation breaks down in detail because secondary mineral assemblages and mass transfer vary widely in response to subtle variations in temperature and (or) fluid flow. Strongly oxidized open systems can evolve relatively low δI8O hydrous oxide-rich assemblages if leaching is efficient (DSDP (Deep Sea Drilling Project) site 396), or relatively anhydrous high δ18O K-feldspar-rich assemblages if plagioclase breaks down (site 417), compared with the clay-mineral-dominated assemblages common in somewhat more restricted flow environments. Zonal distribution and common superposition of various alteration mineral assemblages within rock fragments cause significant small-scale isotopic heterogeneity. A simplified low-temperature alteration scheme involving (a) early-stage Fe–K–Mg clay minerals, (b) middle-stage K-feldspars, oxides, or smectites, depending on the duration, rate, and temperature (?) of fluid flow, and (c) late-stage zeolites and (or) carbonates, is recorded in rather complex time variations in δ18O versus H2O+ for individual rock parcels.