This article traces the development of agency prepositions in Old and Middle English passives, documenting the most important stages of the process and presenting them against a larger background of linguistic changes after the Norman Conquest. The study is based on large, syntactically annotated corpora of Early English, and takes into account all the main agency prepositions, that is, þurh ‘through,’ fram ‘from,’ mid ‘with,’ bi ‘by,’ and of, analyzing their token and type frequency, textual distribution, diachronic trends, collocational preferences, and use in lexically fixed formulas. The results are interpreted within the Construction Grammar framework, with the agency prepositional phrase treated as an emerging construction undergoing a slow and particularly difficult constructionalization process hindered by the restriction of the long passive to the written mode.