Context-based learning approaches have been implemented in school science over the last 40 years as a way to enhance students' interest in, as well as learning outcomes from, science. Contexts are used to connect science with the students' lives and to provide a frame in which concepts can be learned and applied on a "need-to-know"-principle. While effects on interest are coherently reported as positive, they are more diverse regarding cognitive learning outcomes. Hence, the demand for further research on criteria of context-based problems and problem-solving processes has been stated. In this paper, a study is presented investigating students' application of chemical concepts when solving context-based chemistry problems. Tasks for context-based problem solving have been designed systematically, using different combinations of contexts, topics and chemistry concepts in relation to the syllabus. Empirical data were collected using think-aloud interviews where 20 upper secondary students used their chemical content knowledge to solve the problems. The 15 context-based problems raised challenges within organic chemistry where concepts like electronegativity, polarity and solubility had to be applied. The difficulty to differentiate between intra- and intermolecular bonding emphasised in earlier research has also been apparent in this study. Besides the structural formula, which was an important part for the students when solving the tasks, the contextualisation of the problems was often used in the responses; students related their answers to the personal, societal or professional context in different ways. The paper explores the results and gives implications for context-based teaching, learning and assessment.