The most recent evolution of glass-ionomer chemistry is the class of materials known as polyacid – modified resin composites or compomers. These materals are similar to resin – modified glass ionomers in the thy contain all the major components of both polymer – based composites and glass ionomers, with the exception of water. Water is excluded to prevent premature setting of the material and also to ensure that setting occurs only through a polymerization reaction. Limited acid – base reactions are believed to occur once the material is exposed to, and absorbs, water. Although the name compomer implies that the material possesses a combination of characteristics of both composites and glass ionomers, these materials are essentially polymer – based composites that have been slightly modified to take advantage of the potential fluoride – releasing behavior of glass ionomers. The mechanical properties of polyacidmodified resin composites are superior to those of traditional glass ionomers and resin – modfied glass ionomers, and some cases , are equivalent to those of contemporary polymer – based composites.