Abstract
The paper is focused on the process that creates prepositional phrases in Spanish, from the perspective of the theory of grammaticalization and cognitivism, tracking semantic transformations through associative processes such as metaphor and metonymy. After reviewing these theoretical bases, the research follows two particular expressions in the Spanish prepositional phrases inventory: en cabeza de and a la cabeza de. The analysis starts by the study of the central noun, cabeza, in historic dictionaries, followed by a study of their use in the main academic corpus. The aim is to observe how these expressions have changed diachronically to establish when and how they have become fixed phrases in the Spanish language.