The use of preceramic polymers in the synthesis of Sialon ceramics has been scarcely discussed in the literature. In this article we report the production of virtually phase-pure beta'-Sialon ceramics from a mixture of commercially available polysilazanes and gamma-Al2O3 nanopowder, pyrolized in N2 atmosphere in the 1300 degrees C-1600 degrees C range. This approach combines the advantage of embedding nano-sized fillers in preceramic polymers, in terms of their reactivity towards the SiN based ceramic pyrolysis residue, with the complex interactions with residual carbon, also present as a secondary phase in the same ceramic residue. Starting from a polymer (PSZ20) yielding a SiCN amorphous ceramic after pyrolysis, the Sialon phase purity is greatly affected by the residual C content: for an optimized polymer/filler ratio (PSZ20/Al2O3 = 2), beta'-Sialon can be produced possesing only small quantities of quasi(-)amorphous SiC as a secondary phase. Additional improvements based on the partial replacement of PSZ20 with a polymer (PHPS) not containing C in the backbone, lead to the production of pure nanocrystalline beta'-Sialon powders (average grain size of 100200 nm).