The ability of light-bodied elastomeric and reversible hydrocolloid impression materials to spread on a surface was studiedin vitro.The material which most readily coated the surface was a polysulfide, followed by silicone, hydrocolloid, and polyether materials. The influence of surface energy was studied by comparing the spreading of impression materials on high energy (glass) and low energy (PTFE) surfaces. Only for polysulfide and hydrocolloid materials was any effect observed: The glass was more readily coated than PTFE. As a result of an increase in temperature of the elastomeric materials a reduction in the materials' ability to wet the surface was found for silicones and polysulfide, but not for polyether. An addition-polymerized silicone product showed the strongest temperature dependence. A strong negative correlation was found between spreading ability and viscosity of elastomeric materials.