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The pupil offers a window into processes that reflect neuromodulatory brain state. For example, changes in pupil diameter are thought to correlate with exploitatory and exploratory behavioural states. While many studies acknowledge that arousal-based pupil responses are important in controlling behaviour, another important source of information is a signal that carries information about the value of the current behavioural state. Here, we investigated to what extent pupil responses reflect dynamics of behavioural control related to value-based learning. Thirty participants performed an implicit reversal learning experiment where we manipulated the probability of receiving reward or punishment, while recording their pupil size. Coloured cues signalled an upcoming reward or punishment with 80% validity. The contingency between colour and reward/punishment reversed unpredictably, prompting value-based …