Manuscripts in alhamiado (medieval manuscripts in Spanish in Arabic script) are of great interest to historians of the Spanish language because they are a source of data on the pronunciation of Spanish words in the relevant historical period. Of particular value are the records in Alhamiado of the Mosarabs, as there are very few sources in it. Rare Spanish words are written in Arabic script and according to the rules of the Arabic language (in contrast to the alhamiado of Moriscos which is adapted to convey the sounds of Spanish). The orthography of the Alhamiado of the Mosarabs is less developed than that of the Moriscos. In the manuscripts there is an alternation of Arabic letters for Spanish sounds transliterated with the same Latin letter. The scientific novelty of the article lies in the analysis of the alternation of the Arabic letters SIN, SIN AND ZAYN which is not characteristic of the Aljamiado Moriscos to denote the sounds transliterated by Latin x, c and z in the Romance names of the «Archive of the Toledan Mosarabs of the XII and XIII centuries».