Many advantages can be obtained from process analytical technologies to optimize and “green” processes. Utilizing on-line monitoring, a detailed understanding of process variables can be obtained and process improvements can be implemented. In this presentation, precipitation of barium sulphate in a double feed semi-batch stirred reactor was studied. The effects of impeller speed, feed concentration and location on particle size distribution was examined. These findings are compared to results obtained using a single feed semi-batch precipitation. These experiments were conducted using a Mettler-Toledo Lasentec probe which employs Focused Beam Reflectance Measurement (FBRM). A complete analysis of these results will be presented and compared to previous studies given in the literature. The results of this work have been applied to an industrial precious metal reactive precipitation process.