The influence of sodium chloride on the atmospheric corrosion of mild steel has been studied. Samples covered with sodium chloride crystals (8 mg NaCl · cm−2) have been exposed to an SO2-free atmosphere at different relative humidities. The combined effect of sodium chloride crystals on a steel surface and sulphur dioxide in the atmosphere has been investigated at an SO2-supply of 1 mg SO2 · cm–2 · h–1 (1 ppm SO2). The corrosion attack was followed by periodic weighings. The corrosion products formed have been studied using the SEM-technique. On exposure of samples with sodium chloride in an SO2-free atmosphere the extent of corrosion increased with increasing relative humidity from 58% to 90%, interrupted by a sharp minimum at about 87% relative humidity. The SEM-studies showed that tower shaped corrosion products were formed at a high relative humidity while filiform corrosion appeared when the relative humidity was lowered. At 90% relative humidity more corrosion was observed with clean steel samples and an SO2-supply of 1 mg ·cm–2 ·h–1 than with sodium chloride crystals on the surfaces (8 mg NaCl ·cm–2) in the absence of SO2. In the combined influence of sodium chloride on the steel surfaces and sulphur dioxide in the atmosphere a synergic effect was noticed at 90% relative humidity. At 70% relative humidity no influence of an SO2-supply of 1 mg SO2 ·cm–2 ·h–1 on the corrosion of steel samples with sodium chloride crystals on their surfaces could be observed.