The high angular dispersion achieved with the photonic crystal superprism effect as well as dispersive non-periodic photonic nanostructures promise compact wavelength division multiplexing (WDM) devices. An important criterion for the usefulness of such WDM devices is the number of channels that a structure can multiplex or demultiplex. Here two different models are developed for calculating the possible number of channels for a given structure. The first model is based on the assumption that different wavelength channels should propagate in mutually exclusive propagation cones within the volume of the dispersive structure. We call these non-overlapping channels "volume modes." The second model assumes that it is sufficient to spatially separate the different wavelength channels on a single output surface, e.g., the plane of the detectors or output waveguides. Since they are only separated along one surface and not in the entire volume, we name these modes "surface modes." As an example it is shown that a dispersive 200-layer non-periodic thin-film stack can be used to multiplex or demultiplex approximately eight WDM channels. The achievable number of channels is not defined by the dispersion alone but by the product of dispersion and wavelength range over which this dispersion is achieved.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.