Calcium carbonate was precipitated from a supersaturated calcium bicarbonate solution in association with a series of long-chain fatty acid monolayers, and the location of the growing crystals with respect to the monolayer domain structure was determined by in situ analysis using a Brewster angle microscope (BAM). Increase in the chain length from palmitic acid (C16) to triacontanoic acid (C30) strongly affected crystallization, resulting in changes in the crystal polymorphs and morphologies, in the homogeneity of morphology, and in the distribution of crystals over the monolayer surface. Induction times also increased with longer chain lengths, an effect consistent with the decreased rate of carbon dioxide diffusion through monolayers as a function of increasing thickness. While the palmitic acid films supported the growth of high proportions of aragonite, vaterite, and nonoriented calcite, almost exclusive precipitation of an extremely uniform distribution of oriented, regular triangular calcite crystals was produced under triacontanoic acid films. BAM analysis of the system showed that the crystals preferentially nucleated under condensed domains and at domain boundaries. Nucleation at the domain boundaries was attributed to relaxation of the ordered monolayer structure at the domain interface and freedom of the fatty acid molecules to accommodate the growing nucleus. The changes in crystal morphology observed under the different monolayers were attributed to changes in the monolayer structure as a function of chain length.