During initial investigations of a detector for gas chromatography based on modifications of a field-emission argon ionization detector an intriguing selective response phenomenon was identified when iodine vapor was continuously introduced into the detection chamber. The detector was of a simple design consisting of two 1.5 × 0.5 cm concave electrodes mounted in a concentrical ceramic housing. With iodine continuously introduced into the housing via a thermally controlled diffusion tube, certain test compounds of interest, such as valeric acid, triethylphosphate, or aniline, provided analytically significant ionization responses while their hydrocarbon solvents and test compounds went undetected.