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To examine 1) the specificity and number of implementation intentions people form for increasing physical activity, 2) the relation between motivation and the number and specificity of implementation intentions, and 3) the relation between number and specificity of implementation intentions and physical activity change.Content analyses of implementation intentions and prospective design.Secondary data analyses on the implementation intentions 342 adults made in a larger randomized controlled trial. Implementation intentions formed at baseline were rated on specificity. BMI, self-concordance and intentions to increase physical activity were assessed at baseline. Physical activity was assessed at baseline and again two weeks later.Respondents formed on average 2.18 (SD = .84) implementation intentions, but over 30% did not form at least one specific implementation intention. Individuals motivated to increase physical activity formed better, but not more implementation intentions. Regardless of their motivation, respondents who formed better implementation intentions were more physically active two weeks later. The number of implementation intentions people formed was not related to physical activity, except when additional implementation intentions were formed with maximal specificity.Forming implementation intentions of maximal specificity is crucial for behaviour change. Implementation intention interventions might become more effective if individuals are trained to form specific implementation intentions, especially to specify ‘when to act’.