Traditionally, Basque has three voiceless affricates that are different in their place of articulation. More precisely, affricates can have a lamino-alveolar, apico-alveolar, and prepalatal place of articulation and the graphemes used to represent them are respectively. Likewise, Basque has been described as having three fricatives with the same places of articulation as the affricates and they are represented by the graphemes (Hualde 2010; Ortiz de Urbina 2003, Gaminde et al. 2013, Iglesias et al. 2016). However, some speakers merge their Basque sibilants (Hualde 2010, Urrutia et al. 1988, Muxika-Loitzate 2017, Beristain 2018a). This dissertation analyzes the pronunciation of sibilants in a merging variety of Basque and it explores the effects of different factors on the merger, focusing on the effects of Basque-Spanish linguistic contact. In order to explore Basque sibilants, the data of 28 bilingual informants from an interview and a read-aloud task was analyzed acoustically. The role of linguistic contact was explored by measuring informants' degree of linguistic dominance through the Bilingual Language Profile Questionnaire (Birdsong et al. 2012) and by comparing their production of sibilants in Basque and Spanish. In order to explore informants' sibilants in Spanish, the production data of 24 bilingual informants from an interview and a read-aloud task was analyzed. The acoustic measurements used to explore Basque and Spanish sibilants were the Center of Gravity and duration. Separate statistical analyses were carried out to determine the most important factors influencing their production. The data of 14 informants who completed all tasks in Basque and Spanish was used to explore the effects of language contact in detail. The results showed that a higher degree of linguistic dominance in Spanish accelerated the merger of Basque sibilants. Moreover, informants pronounced their Basque sibilants in the front of their oral cavity, which suggested that may not be pronounced with a palatal component as previous descriptions have claimed. The results also showed that there were generational differences among informants, as the merger was more widespread among younger speakers than among older speakers. The analysis of the interviews also showed that speakers who were older than 43 years old used an epenthetic fricative and a palatalized /t/ that younger speakers did not use. With regards to Spanish sibilants, their place of articulation was more stable and it was not affected by informants' BLP or age. The comparison of Basque and Spanish sibilants showed that the majority of Spanish dominant speakers pronounced their Basque affricates with a more fronted place of articulation than their Spanish affricate and this pattern also occurred with their fricatives. Consequently, most informants did not use the same sibilants in Basque and Spanish, but rather they created separate categories for them (as in Simonet 2010). Overall, this dissertation focused on the effects of Basque-Spanish linguistic contact and contributed to the analysis of the current state of the Basque sibilant merger. Moreover, it determined the effects of factors that had not been analyzed in detail previously, such as informants' age. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]