Irritant dermatitis after application of experimental irritants was studied by means of contact thermography. Sixteen healthy persons were patch-tested, using the following irritants: Sodium lauryl sulphate, benzalkonium chloride, nonanoic acid, hydrochloric acid, croton oil, sapo kalinus and sodium hydroxide. A main finding was that croton oil after 24 h caused a warm skin lesion, and sodium lauryl sulphate after 96 h caused a cold skin lesion. This study emphasizes the differences in the skin reactions to different irritants.