This paper explores the decision-making processes of young people aged 13-14?years in 30 consortia across England as they chose their options for Key Stage 4 at a time when a new qualification, the 14-19 Diploma, was being introduced. It draws on data collected as part of a longitudinal national study (January 2008-August 2011) of the introduction and implementation of the 14-19 Diplomas. Few studies of young people's decision-making have considered in detail the role played by the careers education, information, advice and guidance (CEIAG) provided in schools. High quality, impartial school CEIAG provision has historically proved elusive, yet this is often the main source of information for young people and their parents. The introduction of the 14-19 Diploma provided a critical test of the quality of CEIAG in schools delivering that qualification for the first time from September 2008. Through a series of questionnaire surveys of Year 9 learners in 30 Diploma consortia and in-depth case studies in 15 consortia, we investigated young people's decision-making in relation to this new qualification and examined how this was influenced by their school's CEIAG. We found variations in the quality of school CEIAG about the Diploma qualification and, amongst some groups of students, a lack of knowledge and understanding of the Diploma programme of study, and the potential progression pathways. There was also evidence that inaccurate or incomplete CEIAG affected students' levels of satisfaction with their Diploma course. Only 46% of 477 Diploma students surveyed towards the end of their course believed they had made the right choice in Year 9. Although the 14-19 Diploma has now largely been withdrawn, the findings of this study highlight the issues and challenges facing schools as they take over responsibility for the provision of CEIAG from local authorities in September 2012.