Bilingualism and its reference methodology: CLIL are spreading at a very fast pace all through educative systems from some years on. The young status of bilingual programmes leads to little research about how bilingualism is influencing real learning contexts and which factors play important roles in that influence. In this way, this study aims to broaden the empirical base of the field and throw insights about down-to-earth bilingual/CLIL implementation and how it affects to learners' beliefs about the target subjects (EFL and Science/Conocimiento del Medio), about themselves as learners and about the learning context. The method employed is qualitative, over an intact and cross-sectional sample of students in 2nd, 4th and 6th grades within the regional Bilingual Programme in Murcia. Children were interviewed orally in open and recorded interviews guided by 5 key questions about their beliefs. Data was transcribed and coded into representative sections for its analysis. The results coming out of it shown that bilingualism is not always attached to CLIL, and this has a twofold implication: the positive effect of CLIL over children's beliefs and the negative motivational effect of bilingualism when taught with teacher-centred methodologies. This implication has been defined as the double-edged sword effect of bilingualism, which is representative of the higher influence of "methodology" over "bilingualism" on children's beliefs. The following are appended: (1) Guide for interviews; (2) Sheet for interview transcription; (3) Example of collaboration request for parents; (4) Example of collaboration request for schools' directive teams; and (5) Orientation sessions' register.