In this paper I analyze the most controversial thesis of Aristotelian substantialism, namely, that substances cannot be composed of other substances. I call this position the Mereological Limitation Thesis (MLT). I find MLT valid and defend it. My argument for MLT is a version of the argument from the unicity of substantial form. Every substance can have only one substantial form, thus, if some substances compose the objectO, then what binds them is only a set of their accidental forms (relations) (...) and in the result thereofOis not a substance (Ois not informed by a substantial form). I argue against the relativization of the substantiality of forms to the level of composition by showing that substantial forms must be absolutely identity-independent. In the last section I specify the ontological status of parts of substances and argue that they are spatially distributed bundles of accidents of a compound substance itself. (shrink)