One of the subjects that constitute a problem for adult students of mathematics is the understanding of fractions and proportions. These notions are interrelated with proportional reasoning which was the focal point of our study. Using a mixed methods approach, we tried to assess the proportional reasoning of adult students during problem solving. We designed a task sheet with 13 open-ended tasks, each attempted by 30 adult students, and we conducted "clinical interviews" (Ginsburg, 2009) with five participants in order to acquire more data about their ways of thinking. For the task categorisation we used a typology of fraction subconstructs (Kieren, 1980). The results indicate that the adult students' performance was relatively good despite their limited mathematical background. However, they had difficulties with the tasks that required a higher level of thinking.