Fluxless vacuum soldering is widely used for hermetical encapsulation of image intensifier tubes and recently for miniature channel photomultiplier. Extremely clean surfaces are required to obtain good wetting at soldering process of this sophisticated sealing technique. An ultra thin oxide film, covering the liquid solder, can often cause leakage of fluxless soldering seals. The model study of pure indium oxidation in the temperature range from 20 to 400°C in a vacuum at 5x10@super -5@ mbar oxygen pressure was performed using in situ AES measurements. The formation of In@sub 2@O@sub 3@ in the temperature range from 20 to 360°C was established. At temperatures higher than 360°C isothermal dissociation of ultra thin In@sub 2@O@sub 3@ film on liquid In surface was found. We also studied the initial phases of surface oxidation on pure liquid InSn alloy (20at.% In, 80at.% Sn). The surface of liquid solder was exposed to pure oxygen at 250°C. On oxidized InSn surface (1,5 x 10 @super 5@L) only In@sub 2@O@sub 3@ was found. Thermodynamic calculations showed that a mixture of SnO, SnO and In@sub 2@O@sub 3@ was formed at these conditions. The mixed oxides appeared to be thermodynamic unstable near the alloy-oxide interface. The only stable oxide formed on the surface of liquid InSn alloy was found to be pure In@sub 2@O@sub 3@ at the temperatures of 250 ° and below 360°C. At higher temperatures (higher than 360°C) isothermal dissociation of ultra thin In@sub 2@O@sub 3@ on liquid InSn surface following by the process In@sub 2@O@sub 3@ (s)+ 4 In(l)= 3 In@sub 2@O (g) was established too.