Two different methods of image analysis have been used to characterize particulate gel networks quantitatively. The methods have been used to analyse the effect of different preparation conditions on the microstructure of whey protein gels. The microstructure has been characterized at different structural levels by light microscopy, transmission electron microscopy and scanning electron microscopy. The structural parameters have been quantified by digital image analysis and by using a group of experienced microscopists evaluating SEM-micrographs. A stereological approach was used to quantify pore size, particle size and amount of threads within the pores in volume weighted mean volumes. The mode of aggregation was determined by the expert microstructural panel. A 2-level fully factorial experimental design has been used, with heating rate (1-5°C/min), pH (4.6-5.4) and salt addition (0-0.1 mol/dm3) as design parameters. The results showed that the heating rate and the pH had main effects on both the particle size and the pore size. The faster heating rate produced both pores and particles of smaller volume. The mean volume of the particles varied between 0.3 and 1.4 ?3, which could be compared to diameters between 0.8 and 1.4 ?3 if a spherical shape is assumed. The size of pores and particles increased by an increase in pH. Pore volume was found to be affected by an interaction between heating rate and pH. The change in pH had a pronounced effect when the slower heating rate was used. If the voids were approximated with a spherical shape, the diameters varied between 10 and 40 ?, compared with the volumes between 1200 and 29 100 ?3. The interaction effects showed that the heating rate influenced the mode of aggregation at the higher pH 5.4, whereas the salt addition affected the mode of aggregation at the lower pH 4.6. © Oxford University Press.