This paper presents a new technology for oxidizing a
GaAs
1
−
x
P
x
(
x
=
0
∼
1
)
surface by oxygen plasma, produced by high‐frequency discharge in oxygen, whose pressure is of the order of about 0.1 ∼ 1.0 Torr. Growth rate of the oxide film is remarkably high, and can be controlled by the oxygen gas pressure and the high‐frequency field over a range of 50 Aå/sec to 1 μ/sec. The plasma‐oxidized films have complex composition, including polycrystalline
β
‐
Ga
2
O
3
,
GaAsO
4
, and
GaPO
4
. The refractive index of these films measured by ellipsometry reveals a dependence on the substrate material and on film thickness, and also distribution in the film, so that the refractive index changes from a value, corresponding to
β
‐
Ga
2
O
3
at the top surface, to the value approximately equal to that of the substrate material at the oxide‐substrate interface. Such distribution of the refractive index is useful for attaining optical isolation in monolithic optical integrated circuits. Furthermore, this technique has various applications, such as the surface passivation of
GaAs
1
−
x
P
x
devices and dielectric isolation in
GaAs
integrated circuits, fabricated on a semiinsulating substrate.