Aggregation and removal of detrimental substances from peroxide-bleached, fiber-free thermomechanical pulp water by dissolved air flotation (DAF) were studied for different chemicals. A focused beam reflectance measurement (FBRM) instrument was used to continuously assess the aggregation and removal of dissolved and colloidal substances (DCS) during batchwise DAF of process water. FBRM is particularly suitable for assessing the aggregation of DCS during the coagulation process prior to flotation. Differences between flocculants, as well as between different flocculant doses, can be observed. The FBRM results were in good agreement with turbidity measurements and with determination of wood resin and pectic acids by gas chromatography. General trends in the flotation process were also observed. However, monitoring the aggregates during flotation is more complicated due to the interference of air bubbles. To determine the overall flotation efficiency, a two-probe system with measurement points before and aft...