Homogeneous precipitation of zinc sulfide from thioacetamide decomposition in presence of zinc sulfate in acid solution results in the formation of four-scale agglomerates. We present here experimental results relative to the monitoring of several physicochemical (pH, electrical conductivity, concentration in sulfide ions, turbidity) and morphological (agglomerate shape and size) parameters throughout the precipitation process. From these parameters essential characteristics of the precipitation process cat? be determined, especially the supersaturation level and the precipitated product mass. From this information and with the help of microphotographs of samples withdrawn at different stages of the precipitation, it is possible to formulate a new mechanism of zinc sulfide precipitation. In particular, the nature of the different agglomeration scales is elucidated as well their succession in time. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 2553-2562, 2009