AbstractAgglomerates of uniformly sized primary spheres are found in many aerosol and colloidal systems. Such particles have been described as fractal although they possess selfsimilarity over only a narrow range of length scales. Previous work suggests that, for such objects, ideal fractal scaling laws may require substantial corrections. The problem is particularly acute for measurements of particle structure from 2d images. The relation between the structure of an agglomerate and the characteristics of its projection is investigated using simple analytical models and clusters generated by a modified cluster cluster aggregation simulation. The morphology of the simulated clusters is varied by changing the ratio of masses of the colliding clusters. The fractal dimensionsDf3of the simulated clusters were typically 10–20% higher than those measured for the projections,Df2, even thoughDf3>2. This difference decreases as the cluster size increases. It was found that the fraction of primary particles visible in the 2d image is more sensitive to cluster size than to fractal dimension for clusters with less than several thousand primary particle