This study investigates the longitudinal development of language attitudes towards Catalan and Spanish over a five-year period in the transition from primary to secondary education of a sample of students (N = 1,143) in Catalonia and Aragon. Two research questions were investigated: (1) How do language attitudes develop over time? and (2) How do individual and environmental variables contribute to the longitudinal development of language attitudes? The results from a sociolinguistic questionnaire showed that the participants demonstrated positive (for Catalan) and neutral (for Spanish) language attitudes. Regarding their longitudinal development, the findings indicate that the attitudes towards Catalan developed from positive to neutral. These attitudes developed significantly during the transition from primary to secondary and within secondary. Attitudes towards Spanish stayed neutral but significantly decreased over time between primary and secondary education. Results from longitudinal mixed models showed that the variables that accounted the most for attitude differences were environmental, which included the language used in the participant's social network, at home, and with peers. These results are discussed in relation to the development of language attitudes and the impact of individual and environmental factors.