An experiment involving a protocol study of design idea generation is described in this paper. Two designers were used as subjects in a co-operative environment for design. This is less obtrusive to the design process than a single subject thinking out aloud. A framework for macroscopic analysis of cognitive processes in idea generation was developed. It includes identification of activity objects, generation of state diagrams, and statistical analysis of interaction densities. The role of experience and analogies in idea generation was identified and studied. Although the analysis is based on limited data, the idea of non-intrusive protocol study, and the modeling techniques developed could be used for future experiments.