New technological developments offer new collaborative design environments to designers. There is a great interest in these new tools however it is clear that we know very little about how these new design media would affect the way architects collaborate. The study reported in this paper characterizes the collaborative design behaviour of a pair of designers in a prototype which has a 3D modelling tool and 2D sketch pad. The experimental study involves collecting data while a pair of architects work on a prescribed design task. The collected data includes video, verbal protocol data, and screen images. Our study shows that while 2D sketching encourages development of design ideas, the 3D virtual world encourages collaborative modelling of design solutions, in particular designers collaboratively refine the 3D model to visually analyse the design idea as they effectively develop realistic design representations.