The effect of a morphology modifier on the precipitation process of nickel powder was investigated in order to develop an understanding of its mechanism of action. Experiments were conducted on a pilot-plant scale using a 75-L autoclave with modifier dosages in the range of 0.25-5 vol%. Samples were collected from each successive batch reduction within a cycle and the powder was separated from the liquor before being washed and dried for subsequent analysis. The active particle rate processes were identified by transforming the particle size distribution (PSD) data into moments and from the change in surface area as measured by the BET method. Scanning Electron Microscopy (SEM) micrographs of the powder were used to observe the powder morphology and to validate the proposed particle rate processes and mechanism of action of the modifier. Evolution of the first moment (m 0) and third moment (m 3), equivalent to the total number of particles and volume, respectively, indicated that growth and aggregation were the major particle rate processes at a modifier dosage of 0.25 vol%. Breakage became apparent at dosage levels above 0.25 vol%. A decrease in BET surface area was noted in each cycle, indicating the presence of aggregation. The magnitude of decrease in the surface area indicated that the extent of aggregation decreased with increasing modifier dosage. SEM micrographs revealed that the powder was compact and aggregated at modifier dosages between 0.25 and 3 vol% and that loose porous powder was produced at 5 vol%. The modifier was found to inhibit growth, resulting in the formation of weaker agglomerate bridges leading to shear-induced breakage. This led to an increase in the surface area available for reduction. However, the effect of increased surface area in promoting reduction was outweighed by growth inhibition above a modifier dosage of 1 vol%. Thus, the number of attainable batch reductions increased when the modifier dosage was increased from 0.25 to 1 vol% and decreased with further increases in modifier dosage. © 2006 Elsevier Ltd. All rights reserved.