A coordination polymer gel that is self-assembled to form a network structure between a thiacalix[4]arene derivative (L) and Co2+ has been prepared. This gel is capable of selectively changing color in the presence of gases that yield hydrogen chloride upon hydrolysis. The UV-vis absorption spectrum of a coordination polymer gel derived from Co(NO3)(2) exhibits an absorption band at 527 nm and is colored red, indicating the formation of an octahedral Co2+ complex. Treatment with a small amount of volatile gases containing a chlorine atom (VGCl) causes a red shift of similar to 150 nm, resulting in a new strong band with a maximum at 670 nm and a color change to blue. In addition, the red color of the filter paper coated with a Co(NO3)(2) coordination polymer gel changed to blue upon exposure to VGCl, reflecting a change in the coordination geometry. Red and blue colors of single. crystals of Co2+ complexes were obtained from a basic solution. From X-ray crystallographic analysis, the red Co2+ complex corresponds to an octahedral structure, while the blue Co2+ complex reflects the presence of a tetrahedral structure. Thus, the induced color change of Co2+ gel from red to blue upon exposure to VGCl is due to the coordination geometry. The quantitative concentration of VGCl was calculated by employing the RGB histogram available in a smartphone application.