This study aims at gaining an insight on designers' cognitive processes while sketching in digital vs traditional media. Empirical data on design processes have been obtained from protocol analyses of six interior designers solving an interior space-planning problem through media transition. In order to encode the design behavior, a coding scheme was utilized that allowed the inspection of both the design activity and the responses to media transition in terms of the primitive cognitive actions of designers. The analyses of the coding scheme constituents, which are segmentation and cognitive action categories, allowed a comparative study demonstrating the effect of the use of different media in the conceptual design phase. The results showed that traditional media had advantages over the digital media, such as supporting the perception of visual-spatial features, and organizational relations of the design, production of alternative solutions and better conception of the design problem. These results also suggested implications for computer aids in architectural design to support the conceptual phase of the design process.