Studies by means of infrared multiple internal reflection spectroscopy of ultraviolet and visible light polymerized composite resin materials indicated that the degree of conversion below the surface is nearly constant for most of the materials investigated until a depth is reached where the degree of conversion rapidly decreases. The maximum depth of cure has also been measured for some proprietary dental composite resin materials. This investigation has demonstrated that the conversion at different depths and the maximum curing depth are dependent on the composition of the composite filling material, light source, and exposure time. With respect to the activating system it was found that with visible light activated materials larger curing depths were obtained than with ultraviolet light activated materials. Better results with respect to curing depth were also obtained with materials containing conventional inorganic fillers than with those containing so-called microfillers.The limiting factor for total curing depth, and conversion as a function of depth of the proprietary light activated composite resin materials used in this investigation is assumed to be light scattering. Based on a semi-empirical scattering function the influence of particle size and refractive index of the particles and the organic medium on the light scattering were assessed. Maximum scattering occurs when the effective size of the particles is approximately half the wavelength of the activating light. This is the case with a microfill material activated by ultraviolet light.