The electrical conductivity of a two component conductive developer decreases with increasing toner concentration, since the insulative toner particles block carrier-to-carrier contacts. This effect is a function of toner and carrier physical properties such as size, density and the intrinsic conductivity of the carrier particles. However, this effect can be moderated through additional factors such as carrier surface roughness and lubricating toner external additives. In the present paper, these various effects will be illustrated, using an analysis of experimental conductivity data taken on three sizes of model toners.