Two temporary labourers, on piece work without sufficient prior instructions, were employed to clean a house facade. During the work, at first limited to 5-10 minutes, they sustained chemical burns from the hydrofluoric and hydrochloric acids contained in the liquid used to clean the stone. Both workers had been left uniformed about the danger or kind of the material being used, because of their short-time employment. They only sought medical help several hours later. At the areas of contact with the cleansing liquid (hands, lower arms and thighs, in case 1; additionally at first well circumscribed and very painful areas of erythema, developing within a few hours into blisters, in case 2; also erosions in both). Chemical burns by hydrofluoric acid were only elucidated as the cause 24 hours after exposure. Repeat tests (ECG, blood gas analysis, electrolyte levels) provided no evidence of deep-seated burns at the contact areas or systemic poisoning. Radiology of the hand was normal. As there was at first no indicative history, toxic contact dermatitis was suspected and treated topically with Clobetasol-17-propionate. Once the true cause became known treatment consisted of compresses with 10% calcium gluconate solution. All lesions healed completely within 4 and 6 days, respectively. Temporary workers are often insufficiently instructed, despite clear legal regulations. Such lack of informations increases the risk of harm to workers and makes appropriate treatment more difficult.