Cellulose-precipitated calcium carbonate (cellulose-PCC) composite fillers were synthesized through precipitation of calcium carbonate by releasing CO2 from the hydrolysis of ammonium carbonate in the presence of bleached softwood kraft pulp (BSKP) fibrils or carboxymethyl cellulose (CMC). The ratio of generated calcium carbonate to inputted calcium chloride (PCC yield) was much higher with BSKP compared to CMC at 50 degrees C, and much lower with BSKP compared to CMC at 25 degrees C. The characteristics of synthesized composite fillers were analyzed by SEM, XRD, and FT-IR spectroscopy. The results indicate that the average particle size, particle shape, and rate of calcite formation are strongly affected by the type of cellulose substrate and the reaction conditions (versus other polymorphs). Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.