Y0.67Si9Al3ON15 and Dy0.4Y0.3Si8.85Al3.15O1.05N14.95 ceramics were prepared by Spark Plasma Sintering (SPS) with and without heat treatment at 1700degreesC for 7 h or 17 h, and their optical transmittance were investigated over the wavenumber range 4000-1500 cm(-1). The results showed that the assemblages of the SPS-ed samples consisted of single crystallized alpha-sialon phase in both compositions. EDS analysis indicated that alpha-sialon was mainly stabilized by Dy3+ in the multi-cation (Dy,Y)-alpha-sialon composition. The SPS-ed specimens showed relatively high optical transmission properties, and the maximum transmittance reached around 65% at 2800 cm(-1) for 0.5 mm thick specimens of both compositions. The 7 h heat treatment caused the formation of small amount of melilite phase, resulting in non-uniform microstructure and decrease in optical transmittance. Extended heat treatment for 17 h led to more homogenous microstructure and increased the transmittance to some extent. Less melilite was formed in the multi-cation (Dy,Y)-alpha-sialon composition than in the single cation Y-alpha-sialon after heat treatment, and the transmittance of (Dy,Y)-alpha-sialon was also higher than that of Y-alpha-sialon. (C) 2004 Kluwer Academic Publishers.