This paper investigates the entire complex prepositional system of the Spanish language from a grammaticalization perspective. It classifies over 695 forms according to two subtypes: LP and P(D)NP. It also applies Klegr’s (1997) classification of English complex prepositions to the categorization of forms according to seven subtypes of semantic domains: place, time, cause/reason, purpose/goal, criteria, manner and others. While no direct association between formal and semantic subtypes is observed, grammaticalization patterns seen in other languages, such as reanalysis, decategorialization, erosion, persistence, desemanticization, extension, layering, specialization, divergence and analogy, are noticeable. Drawing upon the historically-developmental paths of some of the selected items, this paper argues that both diverse mechanisms and extra-linguistic factors have been entangled in the grammaticalization of complex prepositions of Spanish.