Formalizing the ontological commitment of a logical language means offering a way to specify the intended meaning of its vocabulary by constraining the set of its models, giving explicit information about the intended nature of the modelling primitives and their a priori relationships. We present here a formal definition of ontological commitment which aims to capture the very basic ontological assumptions about the intended domain, related to issues such as identity and internal structure. To tackle such issues, a modal framework endowed with mereo-topological primitives has been adopted. The paper is mostly based on a revisitation of philosophical (and linguistic) literature in the perspective of knowledge representation.