One approach proposed for the exothermic step of the solar Zn/ZnO thermochemical water-splitting cycle is synthesis and hydrolysis of Zn nanoparticles in an aerosol reactor. To date the most common method to quantify the extent of conversion from Zn to ZnO is ex-situ X-ray diffraction analysis of particles collected on a filter placed at the exit of the reactor. The assumption inherent in this approach is that captured particles do not react on the filter because of its relatively low temperature. In the present study, we assess this assumption using data collected on hydrolysis of Zn nanoparticles at temperatures between 360 and 465 K and discuss methods of quantitative analysis via X-ray diffraction (XRD). Zn nanoparticles synthesized via condensation in an argon carrier gas are captured on a filter. Once the particles are collected, steam is introduced to the reactor at controlled partial pressure and temperature. The extent of reaction at the filter is characterized via calibrated XRD. At and below 380 K the reaction is negligible. At 465 K, the rate of conversion is slow at 0.007 ± 0.001 %/min. In most cases, quantifying the extent of conversion from Zn to ZnO in the aerosol by ex-situ X-ray diffraction analysis of particles collected on the filter is an acceptable approach, though care should be taken in both the application of this method and in the quantitative analysis of the XRD data.