In this study, we investigate the semantic properties of the negative prefixes in-and dis-in Lieber's formal framework of lexical semantics. First, we provide a descriptive analysis of these prefixes with a focus on the bases they combine with and the type of negation they contribute in each case. In particular, the two prefixes deliver contrary (e.g. inelastic, dishonest) and contradictory (e.g. inanimate, disengaged) readings on adjectives and privative meanings, i.e. 'lack of X/without X' (e.g. inexperience, disanalogy), on nouns. On verbs, the prefix dis-instantiates standard negative (e.g. disagree), reversative (e.g. disconnect), and pejorative (e.g. dishear) meanings. Second, we propose a formalization of the way the skeletal part of in-and dis-interacts with the skeleton of the bases the two prefixes attach to.