The composition and lattice parameters of several Yb-SiAlON materials are reported. Electron microprobe analysis was used to measure the compositions of each phase, as this technique avoided the peak overlap problems encountered when using energy-dispersive spectroscopy. A refined set of equations relating the composition of Yb-alpha-SiAlON to its lattice parameters is presented. The amount of oxygen substitution (n value) in alpha-SiAlON has an effect on lattice parameters that is almost as strong as the amount of Al and Yb substitution (m value). The alpha-SiAlON phase stability region is shown to extend much closer to the Si3N4-4/3(AlN:Al2O3) line than reported previously.