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The production of BaTi1-yZryO3 (0 ≤ y ≤ 1, BTZ) nanocrystals is known to be challenging due the low reactivity of zirconium precursors. Here we have successfully studied the impact of zirconium on the BTZ particle formation in sub- and supercritical fluid conditions along the entire solid solution. In situ synchrotron wide angle X-ray scattering (WAXS) analyses were conducted in batch at 150 and 400 °C to follow, in real time, the BTZ crystallite synthesis. This approach revealed the complexity behind the nucleation and growth mechanisms of ABO3 nanocrystals, especially toward high zirconium content (more than 50 atomic %). This type of substitution induces, among other things, microstrain within the structure. Moreover, for the cases of BaTi0.4Zr0.6O3 and BaTi0.2Zr0.8O3, the experiments showed the apparition of two crystallite size populations. In the BaTi0.4Zr0.6O3 case, at 400 °C, these two size populations merged into a single one after at least 8 min; in contrast to what was observed for the case of BaTi0.2Zr0.8O3. This is a manifestation of how with increasing the zirconium content the particles become more refractory and in these cases the temperature is not high enough to enable their ripening. It is important to note that this behavior was not observed for particles produced at 400 °C using a flow synthesis method, with a residence time of only 50 s. There, the particles presented a single size population close to the one obtained after 8 min in batch. This thus suggests that for batch syntheses a longer time is required to achieve a similar product quality to the one obtained with a flow process. [ABSTRACT FROM AUTHOR]