The composition of oxide films, formed on stagnant, molten soft solder has been studied. The influence of oxidation time and of solder composition on the oxide films was measured. On clean soft solder SnPb60‐40 a complex oxide film was formed, consisting of a thin outer layer of SnO2, a layer of SnO mixed with metallic lead, and a transition layer to the underlying solder metal. The same type of oxide layer was formed on solder with copper or with nickel. On soft solder contaminated with aluminium, zinc, or phosphorus the contaminants were preferentially oxidized, resulting in an aluminium oxide film, a zinc oxide film, and a mixed phosphorus oxide–tin oxide film, respectively.