Copper oxalate particles precipitated from copper nitrate and sodium oxalate are aggregates of small crystallites. The crystallites seem to be self-organized such that they are very well aligned crystallographically within the volume of the aggregate. The addition of various cellulose derivatives induces different particle and crystal-lite morphologies. Without additives, particles with a cushion-like morphology are observed. Increasing the concentration of hydroxypropylmethylcellulose (HPMC) added to the precipitating agents induces a variation of the shape from low axial ratio forms such as cubes to higher axial ratio rods. The crystallites within these particles show a more elongated shape and smaller size as the HPMC concentration is increased. The polymer additive seems to influence the three steps of copper oxalate precipitation: nucleation, crystallite growth, and aggregation. The presence of HPMC affects the copper oxalate nucleation step whereby more nuclei are created and their sizes decrease as reflected by the crystallite volume (total number of crystallites increases while the precipitate yield is constant). In this paper we describe the characterization of these well-organized aggregates and propose a mechanism for the influence of the HPMC on the crystallite and aggregate shape.