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Calcium carbonate dispersions stabilized by an adsorbed surfactant layer are used to regulate the acidity of lubricating oils for internal combustion engines. Three typical commercial samples have been examined using a combination of X-ray and neutron scattering techniques. The solid active material was extracted from the samples and redispersed in heptane and mixtures of toluene and toluene-d8 and n-octane and n-octane-d18. The results were in broad agreement with the core/shell model proposed by Ottewill [Markovic, Ottewill, Cebula, Field & Marsh (1984). Colloid Polym. Sci. 262, 648–656]. The core radius (~ 12–28 A) was obtained from SAXS data. The polydispersity was about 25% on all samples and the distribution skewed to larger sizes. The hydrogenated-solvent SANS data were too weak to be used to determine the core size. The total particle radius (~ 20–50 A) and hence the shell thickness were obtained by model fitting to the deuterated-solvent and 65% deuterated-solvent SANS data with the core radius from the X-ray data. In determining the best fit, the core density, amount of solvent in the shell and polydispersity were also adjusted to give fits with a shell-to-core-volume ratio and a volume fraction that agreed with the expected values. The SANS data were weaker than expected assuming a simple monodisperse core/shell model with a calcite core. This is consistent with the observed polydispersity and in addition implies diffuse boundaries. Although WAXS (wide-angle X-ray scattering) confirmed that the cores were non-crystalline the best model fits were obtained with a core scattering density not much less than that calculated for calcite. The absolute intensities suggested that some solvent was incorporated into the shells of all the samples and one sample showed a preference for uptake of octane as compared to toluene.