X-ray photoelectron spectroscopy (XPS) analyses of oxides produced by thermal and plasma oxidation of GaAs show that electron cyclotron resonance (ECR) plasma oxidation favors what is thought to be the more desirable (from the point of view of electronic passivation) As+5 oxidation state while thermal oxidation favors the lower oxidation state, As+3 Thermal oxidation produces a Ga-rich oxide, whereas the ECR plasma oxides are nearly stoichiometric. Also, thermal oxidation removes As-(0) during the initial stage. XPS shows that thermal and ECR plasma Si oxides have different structures. In situ and real time ellipsometry studies indicate that thermal and ECR plasma oxidations yield different film growth kinetics for both GaAs and Si. In particular there are no strong substrate orientation effects for the ECR plasma oxidation of GaAs and Si in the initial stage of oxidation. The similarities in our Si and GaAs oxidation results enable models to emerge based on the dominance of the highly reactive plasma generated oxidant species for plasma oxidation and molecular oxygen for the thermal case.