Anionic polyelectrolytes bearing phosphate or phosphonate groups were compared to poly(acrylic acid-co-acrylamide) as reinforcing polymers for paper filled with calcium carbonate. Cationic polyvinylamine, PVAm, was used to promote adsorption of the anionic polymers onto fiber and filler surfaces in the aqueous papermaking suspension. Both poly(acrylamide-co-vinyl phosphonate), PAMVP, and phosphate esters of poly(vinyl alcohol), PVAP, gave stronger paper than did the acrylate copolymers. PAMVP bound soluble calcium ions, resulting in the formation of multichain colloidal-sized clusters. Subsequent addition of PVAm produced dispersed, cationic polyelectrolyte complexes. The polyacrylates bound less soluble calcium and formed weakly scattering clusters. It is proposed that the phosphate and phosphonate groups promote polymer adhesion to the calcium carbonate surfaces, resulting in stronger paper.