Polycarboxylate molecules and oligomers have been investigated as growth modifiers during seeded calcite precipitation. To better understand possible molecular interactions and kinetic effects, additives with different structures and molecular weights have been investigated in this work. All additives show growth modifying effects, albeit less strongly for succinic acid and glutaric acid. This is attributed to a relatively weak interaction with the precipitated particles as well an additive size too small to influence the aggregation phase of the growth mechanism. Poly(acrylic acid) and poly(aspartic acid), on the other hand, led to strong growth modification, with the resulting particles being nanostructured, formed by an assembly of nanosized primary particles and consequently having a high specific surface area. Poly(aspartic acid) showed a stronger growth modifying effect than poly(acrylic acid) at a similar molecular weight and functional group concentration. This was not readily explainable without using molecular dynamics simulations (reported in a separate article), which suggests that the differences originate from the rigidity of the backbone and favorable electrostatic interactions between backbone nitrogen atoms and the surface in the case of poly(aspartic acid).