SiO2 films were deposited by means of the expanding thermal plasma technique at rates in the range of 0.4–1.4 μm/min using an argon/oxygen/octamethylcyclotetrasiloxane (OMCTS) gas mixture. The film composition was studied by means of various optical and nuclear profiling techniques. The films deposited with a low OMCTS to oxygen ratio showed no residual carbon and a low hydrogen content of ∼2% with a refractive index close to thermal oxide. For a higher OMCTS to oxygen ratio a carbon content of ∼4% was detected in the films and the refractive index increased to 1.67. The surface passivation of the SiO2 films was tested on high quality crystalline silicon. The films yielded an excellent level of surface passivation for plasma-deposited SiO2 films with an effective surface recombination velocity of 54 cm/s on 1.3 Ω cm n-type float zone crystalline silicon substrates after a 15 min forming gas anneal at 600 °C.