<![CDATA[This study aimed to describe the implementation of Indonesian Sign Language (BISINDO) within a bilingual instruction model in English vocabulary learning, to examine how the implementation influenced the English vocabulary mastery of deaf students, and to identify supporting and inhibiting factors in its implementation. The study used a qualitative descriptive design supported by descriptive quantitative data from observation, questionnaire, and vocabulary test results. The participants consisted of two English teachers and three deaf students at SLBN Tobadak. Data were collected through classroom observation, student questionnaire, English vocabulary pre-test and post-test, and semi-structured interviews. The data were analyzed through data condensation, data display, and conclusion drawing, with triangulation across instruments and participants. This study investigated the implementation of Indonesian Sign Language (BISINDO) within a bilingual instruction model to support the English vocabulary mastery of deaf students at SLBN Tobadak. Vocabulary mastery is a fundamental component of English as a Foreign Language (EFL) learning, yet deaf students often experience specific challenges because conventional English instruction relies heavily on auditory and written input. This study employed a qualitative descriptive design supported by descriptive quantitative data. The participants were two English teachers and three deaf students at SLBN Tobadak, Central Mamuju Regency, West Sulawesi. Data were collected through classroom observation, student questionnaires, English vocabulary pre-test and post-test, and semi-structured interviews with teachers and students. The data were analyzed through data condensation, data display, conclusion drawing, and cross-instrument triangulation. The findings revealed that BISINDO was implemented as an access language to introduce, clarify, and reinforce English vocabulary. Teachers used visual input, BISINDO signs, written English words, Indonesian explanations when needed, imitation, repetition, and visual corrective feedback. Observation results showed that Teacher G1 obtained 68 out of 72 points, or 94.44%, categorized as very good, while Teacher G2 obtained 63 out of 72 points, or 87.50%, categorized as good. Students’ questionnaire responses were positive to very positive, with an average score of 84.67. Vocabulary test results increased from an average pre-test score of 38.33 to an average post-test score of 75.00, showing an average gain of 36.67 points. The study concludes that BISINDO-based bilingual instruction meaningfully supports deaf students’ English vocabulary mastery when implemented consistently through visual mediation, written-word reinforcement, repeated practice, and corrective feedback.]]>
