The present work determined the phase relations in the Si-C-N-O-R systems, involving SiC-Si3N4-R2O3, Si3N4-SiO2-R2O3, and SiC-Si3N4-SiO2-R2O3 (R=La,Gd,Y) systems by XRD phase analyses of the solid-state reacted samples from powder mixtures of SiC, Si3N4, and R2O3. Subsolidus phase diagrams of these systems were presented and summarized. Within these systems in-situ SiO2 impurity in the powder mixtures often cause the formation of some oxygen-richer rare-earth siliconoxynitrides, J-, K-, H-phases and extends the ternary systems SiC-Si3N4-R2O3 into the quaternary systems SiC-Si3N4-SiO2-R2O3 (Si-C-N-O-R). In these systems, the reaction of partial SiC participation and the formation mechanism of rare-earth siliconoxynitrides are discussed.