Bacterial anchoring to limestone rocks is thought to occur by selective adsorption of biomolecules found in the extracellular matrix, such as polysaccharides. Here we study the adsorbed structure of a model matrix polysaccharide, sodium alginate, at the calcite/water interface using neutron reflection (NR). Sodium alginate was found to form highly hydrated layers extending up to 350 Å into solution at concentrations up to 2.5 ppm (the inflection point of the adsorption isotherm). The adsorption of alginate was driven by dissolution of the calcite surface through complexation of free calcium ions. This was shown using two alginates with differing ratios of sugar residues. Alginates with a higher proportion of guluronic acid (G) have a higher affinity for calcium ions and were found to cause the surface to dissolve to a greater extent and to adsorb more at the surface when compared to alginates with a higher proportion of mannuronic acid (M). Adding magnesium to the high G alginate solution reduced dissolution of the surface and the adsorbed amount. In this work, we have shown that polysaccharide adsorption to sparingly soluble calcite interfaces is closely related to polymer conformation and affinity for free calcium ions in solution.