Liquid 4 He shows a dewetting phase on Cs and Rb at low temperatures. These are the only substrates which are not wetted by 4 He. On Cs the wetting temperature, T w , is about 2 K. Usually one observes a thick metastable helium film remaining on the Cs substrate when either the surface is cooled below T w (so starting from a wet film) or when bulk helium (in the form of a drop or an advancing film) is deposited on the Cs. The contact line of this film is generally very stable and so there is no

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... dewetting. However, we have observed that spontaneous drying of helium on Cs can occur and that contact lines can recede for temperatures close to T w . This exceptional behaviour is analysed and the dependence of substrate roughness is discussed. Furthermore, we have studied the non-wetting 'thin-film' of liquid 4 He on Cs. Far away from coexistence this film grows as a two-dimensional (2D) gas until about one monolayer is obtained, then remains nearly constant for a certain range of chemical potential and, close to coexistence, rapidly forms a thicker 2D-liquid-like film as stable thermodynamical solution.