This paper is a synthesis of works since the 1970s about the adsorption at the mercury electrode of various kinds of solutes, leading together to new insights on various aspects of the double-layer properties and adsorption at metals (especially Hg) from solutions, challenging several hypothesis largely accepted for many decades up to now. The main part is devoted to the double layer in the presence of adsorbed aliphatic species, this kind of compounds being particularly relevant for studying the effects of the competition between electric and hydrophobic interactions. A section is also devoted to the ionic double layer.