This research deals with the loss of distinction between the bilabial and labiodental sounds, /b/and /v/, in Spanish using a theoretical and linguistic diachronical perspective. The phonetic evolution of these sounds and their confusion dates back t0 the roots of the Spanish language in the Peninsula territories, especially in the north, in close geographic contact with the Basque. This confusion was already taking place in Vulgar Latin and, specifically, in the Vulgar Latin spoken in the Peninsula. This lack of distinction evolved slowly until the seventeenth century when both sounds lost their phonemic contrast. Differences between the bilabial plosive /b/, the fricative /ß/, and the labiodental /v/ are not easily represented in the recorded manuscripts from the first centuries of written Spanish. Phonetic changes happening in the spoken language were hardly represented in their written form; or if they were, they did not correspond chronologically to the change. The written language followed a conservative tradition of what was considered the standard written form that did not always match the reality of the spoken language. Therefore studying the confusion of labial consonants, based on old texts and records, could be challenging, and in many ways restricted to a few documents. The main resources used here come from old rhymes and poems, written texts from medieval grammarians, and opinions documented by linguists between the thirteen and seventeenth centuries. Thanks to these resources (useful but very limited), some chronological benchmarks have been established to indicate the initial confusion, the expansion of this confusion, and the final loss of phonemic distinction.