This paper investigates the effect of metal chloride solutions introduced as mist on small-scale diffusion flames of n-heptane. The heptane bums in a glass cup surrounded by a glass chimney. An ultrasonic atomiser produces fine mist (D50=8.2µm) delivered through the glass chimney with air flowing at 48L/min. We study metal salts including chlorides of Li, Na, and К from the alkali metals, Mg and Ca from the alkaline earth metals, and Mn and Fe from the transition metals, as well as pure water. It is shown in the experimental results that, the pure water mist extinguishes cup-burner flames at a critical loading of 174g mist/m3 air. This coincides with the critical n-heptane evaporation flux of 0.0083kg/m2s. The calculations based on the heat extraction from flames by water mist indicate the critical loading of 149g mist/m3 air. With respect to their suppression effectiveness, the alkali metals rank in the order K, Na, Li, with the corresponding critical mist loadings of 132, 149, and 162g of 0.4M solution/m3 air, respectively. The results for the alkaline earth and transition metal salts are inconclusive since these chemicals hinder the performance of the atomiser. With the careful re-calibration of the atomiser, we determine that, the manganese solution extinguishes flames at a mist loading of 139 g/m3, which ranks this metal closely behind potassium in terms of flame extinguishment efficiency.