Liquid films spreading spontaneously on solid surfaces may exhibit very different behaviour, depending on the surface energy, atmospheric relative humidity, presence of temperature or concentration gradients, etc. In homogeneous situations, wetting liquids build molecularly thin films, which can be conveniently studied at the edge of spreading microdroplets. Spatially resolved ellipsometry is the relevant technique for this study. We present new results showing the various roles of atmospheric humidity in the spreading of polydimethylsiloxanes, silane derivatives and trisiloxane surfactants (which do not wet completely, but build a well organized bilayer on hydrophobic surfaces). In the presence of surface tension gradients, induced by temperature or concentration gradients, the films are much thicker and grow more rapidly. Contact line or interfacial Rayleigh-like instabilities frequently occur. Microscopy or interferometry are the relevant techniques for studying these cases. We present examples of such behaviour in the systems ethanol–water and ethylene glycol–non-ionic surfactant C12E10.