Objectives This study aims to evaluate the effectiveness of a "blended" workplace health-promotion intervention, alongside identification of key components beneficial for future implementation strategies. Methods Within a cluster randomized controlled trial, 491 employees at increased risk of cardiovascular disease were allocated to the limited (N=217; 9 clusters) or extensive (N=274; 8 clusters) intervention. The extensive intervention consisted of motivational interviewing (MI) within the framework of a web-based health risk assessment (HRA), a blended care approach. The limited intervention received solely the web-based HRA. Occupational health physicians (N=21) within three organizations delivered the intervention. Implementation components investigated included: HRA (reach and participation), newsletters (percentage read), and MI sessions (number and quality). MI quality was determined by scoring audiotaped MI sessions, using the MI treatment integrity code. After 6 and 12 months, effects on participation in health-promotion activities and its associations with components of implementation were determined by mixed-effects models. Results Over 80% of employees participated in health-promotion activities, with an additional 8% in the extensive compared to the limited group. In the extensive intervention, those with more or better quality MI sessions were more likely to participate in health-promotion activities. Increased MI quality was associated with sustained participation. Conclusions This study suggests that participation in health-promotion activities can be increased by adding MI to a web-based approach and improving the quality of the MI delivered. Interventions with MI should include optimized delivery and quality of MI sessions.