Solid state electrolytes are supposed to be key enabling components for the development of the next generation lithium ion batteries. One of the promising materials for Li-ion conduction is Li1 + xAlxTi2 - x(PO4)(3) (LATP) due to its high ionic conductivity and stability against water, allowing water based pitduction processes. During synthesis and sintering of LATP, secondary phases such as AlPO4 and LiTiOPO4 may occur. These phases can lead to different properties after sintering. This investigation contributes to a better understanding of the influence of LiTiOPO4 on LATP and LiTi2(PO4)(3) (LTP), which is an aluminum-free analogue to LATP. Therefore, powder mixtures containing 5 wt% and 10 wt% LiTiOPO4 were produced. The powders and the sintering behavior are analyzed by differential thermal analysis and dilatometry. Phase composition Was examined by X-ray diffraction and impedance spectroscopy was performed to study the ionic conductivity. The microstructure of Sintered specimens was analyzed by scanning electron microscopy and correlated to both, density and ionic conductivity. In this work it is shown that the effect of a specific addition of LiTiOPO4 strongly depends on the amount of AlPO4 in the system. The addition of LiTiOPO4 to LTP lowers the sintering temperature and results in a more homogeneous microstructure. In contrast to this, the addition of LiTiOPO4 to LATP increases the sintering temperature and, if added in excess, causes abnormal grain growth. Still, the addition of a small amount of 5 wt% LiTiOPO4 to LATP improves conductivity. (C) 2016 Elsevier B.V. All rights reserved.