The complete dielectric tensor for single crystals of polymerizable pTS diacetylene was determined as a function of temperature and monomer-to-polymer conversion. A pronounced elongation of the ellipsoid of permittivity is observed in the direction of growing polymer chains, whereas the cross section perpendicular to this direction is almost isotropic, being affected by the polymerization to a minor degree. To within experimental error two principal axes of the tensor coincide with the crystallographic axes, the orientation of the tensor being independent of temperature and polymer content. Phase transitions in monomer, polymer and mixed monomer-polymer crystals are visible on the ϵ( T) dependences. The changes of permittivity outside the region of phase transitions may be understood within models based on concepts of the Lorentz or Onsager local fields. In the vicinity of phase transitions the dependence of permittivity on temperature and polymer contents is explained employing the Landau-Devonshire-Kittel theory for antiferroelectric-type phase transitions.