There is disclosed a method of operating a colorimetric gas detector system that comprises a substrate (1) bearing a material (12) that can react with a target gas to produce a change in the wavelength of radiation absorbed or transmitted by the material (“color-change material”). The method involves: a) applying onto a region (15) of the substrate that includes color-change material a chemical of predetermined concentration that reacts directly or indirectly with the color-change material to produce a change in the wavelength of the radiation absorbed or transmitted by the material; b) detecting the radiation absorbed or transmitted in said region (15) at a wavelength absorbed or transmitted by the reaction product of the color change material with the chemical, and c) generating a signal in accordance with the amount of radiation detected at the second wavelength, said signal being dependent on the amount of color-change material on the substrate. The signal can be used to correct the reading of the color change material when exposed to target gas to compensate for varying amounts of color-change material on the substrate.