While trying to complete arduous tasks (e.g., emails, grading) our attention is often mired by the desire to disengage. Opportunity cost theories of mental effort argue that rather than this ‘sense of effort’ being a cognitive limitation, it is an adaptive signal which repels us from unrewarding tasks towards worthwhile alternatives; in short, this signal ensures our cognitive resources are not spent on fruitless pursuits. The current work tests the primary predictions of the opportunity cost theory of effort: that our phenomenology during a cognitively demanding task (sense of effort and boredom), and subsequent on-task behaviour (response times and accuracy), is affected by the value of the available alternatives. Over three experiments, manipulating both the extrinsic value (i.e., monetary reward) and intrinsic value of alternative tasks (i.e., how enjoyable the task is) we find little evidence in line with the predictions of the opportunity cost account. In Experiment 1 we observe no effect of the extrinsic value of an alternative task on participants subjective ratings or on-task behaviour during a primary task. Furthermore, in Experiments 2 and 3, while participants’ subjective ratings of a primary task (e.g., sense of effort and boredom) are affected by the intrinsic value of an alternative, we observe no commensurate changes in participants’ accuracy or response times.