Although calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved. Here, we investigate the early stages of calcium oxalate formation in pure and citrate-bearing solutions. Citrate is used as a well-known oxalate precipitation inhibitor; moreover, it resembles the functional domains of the biomolecules that modulate biomineralization. Our data suggest that calcium oxalate forms after Ca2+ and C2O42- association into polynuclear stable complexes that aggregate into larger assemblies, from which amorphous calcium oxalate nucleates. Previous work has explained citrate inhibitory effects according to classical theories. Here we show that citrate interacts with all early stage CaC2O4 species (polynuclear stable complexes and amorphous precursors), inhibiting calcium oxalate nucleation by colloidal stabilization of polynuclear stable complexes and amorphous calcium oxalate. The control that citrate exerts on calcium oxalate biomineralization may thus begin earlier than previously thought. These insights provide information regarding the mechanisms governing biomineralization, including pathological processes (e.g., kidney stone formation).The formation mechanism of abundant calcium oxalate biomaterials is unresolved. Here the authors show the early stages of calcium oxalate formation in pure and citrate-bearing solutions by using a titration set-up in conjunction with solution quenching, transmission electron microscopy and analytical ultracentrifugation.