Research on the history of the Old Spanish imperfect paradigm in -ie has yet to provide a consistent account of the morphological and phonological factors that contributed to the eradication of this paradigm from Spanish verb morphology. A detailed analysis that takes into account the interaction of factors that played an integral part in the evolution of other facets of Old Spanish morphology and phonology may shed more light on the disappearance of the -ie paradigm and the eventual restoration of its etymological competitor in -ia. The current investigation compares data from four Old Spanish texts with the purpose of examining the impact that five linguistic variables had on the Old Spanish imperfect between the late 13th and early 15th centuries. The results show that different sets of variables are responsible for variation in each text, indicating that -ie ~ -ia alternation was not attributable to the impact of one linguistic factor, but rather to that of a combination of multiple factors through time. Furthermore, the important discovery is made here that the development of the Old Spanish imperfect is best understood from a more comprehensive framework of Spanish diachrony.