Free settling tests, small-angle light scattering, microtome-slicing techniques, and confocal laser scanning microscopy were performed to examine how the cationic flocculation or freezing and thawing affected the floe structure. The floe size, internal pore size, mass fractal dimensions determined from free-settling test or small angle light scattering test, aeral porosity, boundary fractal dimension and Sierpinski carpet fractal dimension of pore boundary from 2D slices, and the volume porosity, compactness, and the pores' box-counting fractal dimension from 3D reconstructed image, were estimated and compared. Cationic flocculation would produce large floes with internal pores of shape resembling a long "tube" with rough surface. Freezing and thawing would produce floes with internal pores with lower aspect ratio and a smoother boundary. (C) 2004 Elsevier Ltd. All rights reserved.