The aggregation process in which basic units (atoms, particles, small agglomerates, etc.) stick together has been intensively studied in recent years [l to 31. Special attention received the kinetic growth processes such as diffusion-limited aggregation in which the structural rearrangements of growing aggregates are not allowed, because they lead to unusual morphologies of aggregates. This process dominates in the earliest stages of the phase separation and it eventually is replaced by the nucleation or spinodal decomposition processes [4, 51. Moreover the formation of aggregates by the kinetic growth process is observed in systems being far from equilibrium, e.g. for deep quenches. The understanding of the kinetic growth of aggregates is thus important from a scientific point of view. The intensive study of this process is also motivated by the need to develop new materials [6]. The unusual morphologies of materials with non-equilibrium structures are connected with unexpected combinations of their chemical and physical properties. Recently properties of rather small metallic aggregates embedded in a matrix or in porous media attracted considerable attention. They exhibit characteristic features that are not found in pure materials, e.g. they show a pronounced peak in the light absorption spectrum, what found applications in selective absorbers of solar energy [7]. The observed optical absorption is due to excitations of surface plasmon modes of conduction electrons, what strongly depends on the morphology and the size of an aggregate. In order to determine the relation between the morphology and properties of an aggregate, it is necessary to define quantities well describing the morphology. The quantification of the morphology of aggregates attracted considerable attention in recent years. Nowadays the morphology of small aggregates can be observed directly by using electron microscopes [ 1, 71. In order to characterize complex morphologies the concept of the fractal dimension was introduced. In [ 11 different methods of calculation of the fractal dimension of small aggregates by analyzing their digitized images are given. However, during the growth process the fractal dimension of an aggregate increases [8], although the growth process remains the same. This is due to the decreasing contribution of the surface part of the growing aggregate in its overall structure. In recent years the non-orthogonal wavelet transform [9] has been used for the characterization of the geometrical complexity of numerical and experimental aggregates [lo, 111. In this note we characterize the morphology of an aggregate during the growth by quantities obtained from decomposition coefficients of the digitized image of an aggregate into an orthogonal wavelet basis. The orthogonal wavelet basis is constructed from the analyzing wavelet ') E-15706 Santiago de Compostela, Spain. ' ) Permanent address: