[eng] The aim of this work is to analyse the seseo (confusion between [s] and [θ]) in Mallorca from a diachronical point of view. This seseo appears in the context of the contact of Spanish and Catalan in this island. In order to carry out this analysis, we will examine documents written between the 17th and 19th c. from the Corpus Mallorca, a corpus of documents written in Spanish by bilingual speakers. Thus, we will study, on one hand, the extent of seseo and the inverse spellings (<c, z>, ); on the other, the possible conditioning factors of this phenomenon from a phonetic and lexical point of view.