High quality barium strontium titanate (BaxSr1_TiO3 with 0 <= x <= 1-BST) nanoparticles can be synthesized using supercritical fluids technology. Well crystallized particles of 20 nm with a narrow size distribution were produced in a single step. The reaction is achieved at relatively low temperature (T< 400 C) and in tens of seconds. The combination of in situ synchrotron wide angle X-ray scattering (WAXS) and ex situ analyses in the form of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM) leads to an understanding of the influence of the substitution of barium cations with strontium ones on the BST nanoparticle growth. A correlation between particle size, density of -OH groups at the surface of the particles and BST composition is exhibited; the higher the -OH density and the lower the strontium concentration, the larger the particles. This confirms that the formation of BST nanoparticles in supercritical fluids is governed by a sol-gel mechanism.