The cause of water sensitivity in wellbound manufactured 20th Century oil paints is investigated. The sensitivity was characterised by swab rolling tests with water and saliva undertaken by four conservators. Paint from <i>Les Animaux</i> by Karel Appel was studied together with samples of contemporary manufactured artists’ oil paints from three manufacturers to which metal stearates, hydroxides, fatty acids were added. Paints containing ultramarine were the most sensitive and separation of pigment and medium was observed in both ultramarine paint from <i>Les Animaux</i> and in paint samples. Paints with cadmium pigments and organic pigments were also generally water sensitive, whereas zinc white paints were not in any of the tests. Light ageing, and the addition of 2% or 5% zinc and aluminium stearate to the paints marginally increased the water sensitivity, whereas aluminium hydroxide had no effect. There is evidence to suggest that failure of dispersant and subsequent aggregation are critical; no indication of chemical degradation of stearate was found after ageing.