An optically transparent xerogel encapsulating Cu(II) acetate is fabricated to detect mustard gas. (HD) analogues via 4 charge-transfer mechanism. A fast response (color change from sky blue to canary yellow) is observed for the chlorinated sulfide, and is accompanied by an absorption band at 370-420 mn. MO calculations revealed that the chlorinated HD analogue displays a charge-transfer transition extended from sulfur to chlorine atom. A 1:1 complex of Cu(II)/HD analogue is preferred. For a colorimetric sol-gel detector prepared, at pH 3, the detection limit of HD analogue is calibrated at 0.03 mul per 1.5 ml sensor volume. (C) 2002 Elsevier Science B.V. All rights reserved.