We propose a structure comprising a defective one-dimensional planar binary photonic crystal (PC) to realize a demultiplexing operation for dense wavelength division multiplexer (DWDM) transmission systems. The unit cell of the crystal consists of ZnSe and ZnS thin films, and the defective layer is ZnS_xSe_{1  −  x}, (0  ≤  x  ≤  1). Transmittance for the structure was numerically investigated using the transfer matrix method. The selecting mechanism of the output wavelength was based on selecting the appropriate sulfur concentration (x value) in the defective layer of the PC. We used six different sulfur concentrations to demultiplex six different output channels. The line width of the output channels was about 0.13 nm with a quality factor of 12,199. The average crosstalk between the adjacent transmitted channels was about −22.16  dB. The overall dimension of the proposed demultiplexer was about 108.4  μm, which practically makes the demultiplexer easily integrated into photonic circuits. Our results show that the proposed structure has all of the required characteristics to demultiplex the C-band signals with a 0.16 nm separation, which corresponds to the ITU grid for DWDM.