Phosphate ester used as a dispersant for BaTiO3 tape-casting slips leaves a certain amount of residual phosphorus which modifies the sintering behavior, microstructure, and electrical properties of the sintered ceramic. Dilatometric measurements together with SEM and TEM study of the microstructure revealed that residual phosphorus located at the surfaces of the BaTiO3 particles inhibits grain growth during the first sintering stage. High density materials with homogeneous fine-grained microstructure were sintered below 1300 degrees C. Higher sintering temperatures led to grain growth and Ba-3(PO4)(2) second phase was formed. For the samples with a homogeneous fine-grained microstructure, permittivity values as high as 4000 were measured, while dielectric losses were around one order of magnitude lower than those for the phosphorus-free material. On the other hand, when second phase was observed, a marked increase of the dielectric losses was detected. Based on the microstructure characteristics of the samples and the analysis of the grain boundary conductivity by complex impedance spectroscopy, it is proposed that changes in the local concentration of P5+ at the grain boundaries are responsible for the change of the grain boundary conductivity. The charge compensation mechanism would change from cation vacancy to Ti3+ formation depending on the local P5+ concentration at the grain boundaries.