The physical properties of coatings depend on several parameters which are decisive for the function and the coatings quality. Parameters such as particle temperature, size, rate and velocity in addition to a careful substrate preparation are well known to determine the adhesion and mechanical properties of the layer. These particle parameters can be measured in situ using various diagnostics. Nonetheless, another quality-determining parameter is the degree of oxidation which could not be measured in situ (up-to-now) by established diagnostics and must therefore be qualitatively derived from the above mentioned quantities or subsequently be measured by means of destructive methods. Within this work, a diagnostic method to determine in-flight particle oxidation is presented. The method is based on tomographic two-color pyrometry, which allows the calculation of spatially resolved temperature and intensity distributions of the entire particle plume. By additionally measuring the particle velocities and particle sizes the surface emissivity of the particles along the spraying direction can be calculated, which in turn allows quantitative conclusions on the degree of particle oxidation. The method is validated by investigations on the wire arc spraying process. Here different spraying parameters are varied, such as voltage, wire feed rate and gas pressure. Another atomizing gas is also used to reduce particle oxidation and further validate the diagnostic method. The results are correlated with oxide contents in finished coatings determined from microsections.