Large size and high quality single crystals of Nd$_{2−x}$Sr$_x$NiO$_{4+δ}$ and Pr$_{2−x}$SrxNiO$_{4+δ}$, with selected composition (x = 0.0, 0.1 and 0.5), were grown by a floating-zone technique using an image furnace. We found that even small deviations from the ideal cation stoichiometry led to either NiO segregation or formation of Ni-poor Pr$_4$Ni$_3$O$_{10−x}$, or RE$_x$O$_y$ intergrowth phases, rendering the title compounds unstable even under ambient conditions. Related to potential applications as membranes in SOFC, we importantly found Nd$_{2−x}$Sr$_x$NiO$_{4+δ}$ and Pr$_{2−x}$SrxNiO$_{4+δ}$ powders, obtained from ground stoichiometric single crystals, to be stable in air at high temperature (1000 °C), contrary to the partial decomposition observed when synthesized as polycrystalline powders by classical solid state synthesis. The presence of NiO/Pr$_4$Ni$_3$O$_{10−x}$ or RE$_x$O$_y$ intergrowth phases are discussed as a drawback limiting high temperature stability for Pr$_2$NiO$_{4+δ}$. Grown single crystals were characterised by neutron and X-ray diffraction as well as by electron microscopy (SEM/EDS), revealing their excellent quality in terms of composition, homogeneity and crystallinity. For each crystal, the oxygen content was determined by thermogravimetric analysis in a reductive atmosphere. Transmission electron microscopy highlighted a complex incommensurate structure for Pr$_2$NiO$_{4.25}$ and Nd$_2$NiO$_{4.25}$ with a 2D modulation vector related to oxygen ordering.Read less <