This paper presents a method of evaluating the influence of future alternative interconnection technologies at system level. A substitution algorithm has been developed that replaces single critical paths in complex CMOS systems by optical interconnects so that the effect of each substitution on overall system performance can be calculated, in order to decide whether this substitution is feasible under such technological constraints as cost, speed, area, and power consumption. In this paper the focus is on determining the maximum clock frequency, as a typical frequency for data exchange, which is at least up to now the upper frequency limit for processing elements. To attain the optimum ratio of electrical to optical interconnections for systems conceived as hybrid (i.e., containing both types of interconnections), the model takes into account the influence of every single substitution on the electrically wired rest of the system. Thus an optimum interconnection technology can be determined even before a system of given complexity is designed.