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Line managers' behaviours are important during implementation of occupational health interventions. Still, little is known about how these behaviours are related to intervention outcomes. This study explored the relationship between line managers' intervention-specific transformational leadership (IsTL), intervention fit (the match between the intervention, persons involved, and the surrounding environment), and change in intrinsic motivation and vigour. Both direct and indirect relationships between IsTL and change in intrinsic motivation and vigour were tested. Ninety employees participating in an organizational-level occupational health intervention provided questionnaire ratings at baseline and after 6 months. The results showed IsTL to be related to intervention fit and intervention fit to be related to intrinsic motivation. Using intervention fit as a mediator, the total effects (direct and indirect combined) of IsTL on change in intrinsic motivation and vigour were significant. In addition, IsTL had a specific indirect effect on intrinsic motivation. This study is the first to use IsTL as a measure line managers' behaviours. It is also the first to empirically evaluate the association between intervention fit and intervention outcomes. By including these measures in evaluations of organizational-level occupational health interventions, we can provide more informative answers as to what can make interventions successful.