Observation of the diffusion of quinoline inside PVC grains by microscopy represents a very sensitive method of PVC powder differentiation. The kinetics of microdiffusion is measured as a function of time and temperature (exponential function) by the fraction of impregnated grains of the powder which varies, on a broad range, as a function of resin properties, which are mainly influenced by the conversion factor. Gaussologarithmic representation of the phenomenon gives a Galton's straight line, the slope of which is related, more or less, to the homogeneous character of microdiffusion in the powder. That aspect is described in relation to the problem of formation of “fish eyes.”