AbstractSodium aluminosilicate has been precipitated in-situ as filler on hardwood bleached kraft pulp fibers using papermaker’s alum and sodium silicate. The filler was produced in two ways, first in the absence of the fibers and second in the presence of fibers, i.e. in-situ precipitation of filler. The filler produced in absence of fiber was then added to the pulp slurry. Various pulp and paper properties were compared for direct loading of market filler, fresh filler loading, and filler prepared in-situ with fibers. In-situ precipitation technology provided paper with significant improvements in various properties of paper as compared to fillers directly added to the stock. Bulk and stiffness of the handsheets prepared with in-situ precipitation were much higher as compared to those of sheets prepared with fillers directly added to the pulp. There was no appreciable increase in brightness and whiteness of paper with in-situ precipitation, as an appreciable proportion of filler was precipitated inside the fibers. In-situ filler loaded pulps showed a higher filler retention value as compared to directly filler loaded pulps, as a high dose of retention aid was needed with the fillers directly added to the stock.