This paper describes grammaticalization of the complex prepositions, because of and by reason of. By cause of is a loan translation of the French par cause de and by reason of followed [PREP+N+PREP] construction, by cause of through analogy. This paper also shows evolution of particular meanings and semantic changes, based on semantic designations of the causal connectives provided in Oxford English Dictionary (2nd ed.; 1991). Five major mechanisms are invoked here to explain such semantic changes, i.e. metaphor, metonymy, generalization, subjectification, and anthropocentricity. This investigation shows that semantic change is a complex process in which multifarious factors and mechanisms interplay. Finally this paper addresses different aspects of the current usage as well as the distributional characteristics of these complex prepositions by using the 100-million word British National Corpus. This study shows that given the distributional characteristics of these complex prepositions, the current usages of the prepositions are still restricted and affected by semantic persistence.