Previous scholarship on Arabic prepositions typically has presented these as a static closed class of words. Inevitably, such a treatment does not take into account the diachronic development of prepositions into new functions in syntax, semantics and discourse. The present study applies grammaticalization theory to the analysis of prepositions and subordinators across varieties of Arabic. It goes beyond the traditional single-word focus and treats prepositions as parts of multiword complexes. Drawing upon a sizeable base of authentic historical and present-day Arabic data, it presents a rigorously descriptive and quantitative analysis of evolutionary processes involving prepositional forms and subordinators.