Vivid mental imagery during imaginal exposure is generally assumed to promote fear reduction. Nevertheless, vividness can increase distress and it is not clear to what extent fear reduction during imaginal exposure depends on imagery vividness of the fear-provoking stimulus. We examined the link between mental imagery vividness and the reduction of fear in imaginal extinction, an experimental analogue of imaginal exposure. In imaginal extinction conditioned fear, as measured with skin conductance, is reduced through exposure to mental imagery of the conditioned stimulus. We investigated if the reduction of conditioned fear was moderated by task-specific vividness of the conditioned stimulus during imaginal extinction. Fear responses within the extinction session were compared between participants reporting higher versus lower task-specific vividness. Remaining fear responses were also compared between vividness groups (high vs low), 24 h after imaginal extinction in a reinstatement procedure. The study took part over three consecutive days, with fear conditioning to visual stimuli on day 1, imaginal extinction on day 2, and a fear reinstatement procedure, again to visual stimuli, on day 3. Skin conductance is used to measure fear responses. Participants' were allocated to receive conditioning, extinction and reinstatement with either complex or simple stimuli. During imaginal extinction, imagery of each experimental stimulus was prompted through different verbal instructions.