The paper describes the size, density and strength of flocs gained from a humic acid suspension coagulated with aluminium sulphate over a range of dose and pH. Flocs were generated on a continuous flow basis in an oscillatory mixer. Particle size measurements were gained using CCTV and image analysis. From this, a maximum floc size (d95) was identified. A second series of experiments examined the floc sizes of the bulk precipitate alone under equivalent conditions. It was found that the overall floc sizes of the two suspensions were broadly similar, suggesting that the floc strength was dominated by a common bonding mechanism irrespective of the presence of the humic colloids. Some features of the size distribution in response to dose and pH were attributed to the influence of surface charge. Upper floc sizes were proportional to the Kolmogorov length (η) with d95/η ∼1. For typical levels of mixing, the floc strength was estimated to be on the order of 0.1 N m−2. By analysing the breakage kinetics in energy terms and relating the strength to the density of individual floc, a physically-based structural model was developed to explain the response of the floc size to the prevailing state of mixing.