Diffusion models of radical technologies are often based on an epidemic structure developed on the Bass principles of generic external and internal communications. However, in most cases such processes involve more complex communication and decision mechanisms. The diffusion processes should account for interdependences with other innovations and also the substitution mechanisms with regard to the technology in place. Substitution must be understood with a much broader scope than the analytical view of the Fisher and Pry model. As many diffusion authors have acknowledged (but partially excluded from their models), diffusion is a social process with innovation moving through interpersonal networks. We propose a model that accounts for the dynamics of social factors in technological substitution. Based on the System Dynamics methodology, our model disaggregates most of the communication structure and individuals’ characteristics that are implicitly embedded in traditional diffusion models. Our discussion starts with the characteristics of radical technological innovations. We then proceed with the theoretical basis of our social aggregation approach by presenting Kelly’s personal constructs system theory, outlines of social psychology, decision making under uncertainty, and the interdependences between innovations’ characteristics and human behavior. We also make an attempt at considering both sides (consumers adoption / competitors imitation) of a substitutive diffusion. Brice Dattée's research is funded by the National Institute of Technology Management in Ireland.