Creep characteristics of four light activated composites of different composition were determined at 37 and 50°C. The creep properties were compared with the composition of the materials. The microfill material had higher creep values than those with conventional inorganic particles; possibly due to the inhomogenous distribution of the agglomerated inorganic microfill particles. Furthermore, the fraction of inorganic particles was less than in the conventional composite materials. One conventional composite, however, had nearly the same creep values as the microfill material. The content of a large monomer molecule and poor conversion in this conventional composite material may have contributed to a plasticizing effect. The composite material with the lowest creep values had an oligomer/monomer system leading to short branches with pendant methacrylate groups after polymerization and relatively good conversion.