An article may include a substrate and a coating system on the substrate. The coating system may include an environmental barrier coating (EBC) layer and a CMAS resistant layer on the EBC layer (e.g., as the top coat of the system). The CMAS layer includes a rare-earth (RE) monosilicate composition including a plurality of RE metal cations, wherein RE monosilicate composition is configured to react with CMAS to form a reaction product including a RE apatite phase with a RE2O3.SiO2 composition, wherein the RE of the RE2O3.SiO2 composition includes at least one of the plurality of RE metal cations of the RE monosilicate.