This research concerns the design, implementation and evaluation of a blended training course for interpreter trainers. Some of the complex issues pertaining to professional development in a rich web-based learner-centered environment are addressed. Findings confirm a socio-constructivist design within which participants developed the expected skills and knowledge specified in the learning outcomes. Faculty, acting as cognitive and practitioner models, viewed this new teaching experience positively. The use of a design-based research approach led to the creation of a new design framework and a set of design rules readily usable in similar adult training contexts. Future research will investigate the consequences of this professional development sequence.