Taking Cognitive Grammar as its framework, this paper deals with the grammaticalization of prepositions into prefixes through the analysis of the role that metonymy and metaphor and metonymy in interaction play in this process. It is contended that the integration of the prefix with a base, as allowed by a metonymic shift, can occur directly should both elements share the same domain. By contrast, if there are two domains involved, a prior metaphorical operation is required in order to neutralize the ontological differences between the two domains. With this hypothesis in mind, the author focuses on the analysis of two Polish prepositions (i.e. "na" meaning 'on' and "do" meaning 'to') which have not only evolved into prefixes, but have also altered the semantics of the verbs with which they are coupled, namely, "pakowa?" (i.e. ‘to put things into a container’) and my?le? (‘to think’).