The solid solution limit of Pb1−x(La1/2K1/2)xTiO3 was determined in the composition range of 0 ≤ x < 1. The compounds of Pb1−x(La1/2K1/2)xTiO3 were indexed in a tetragonal system for 0 ≤ x ≤ 0.4 and indexed in a cubic one for 0.45 ≤ x < 1 at room temperature. A tiny amount of La2Ti2O7 pyrochlore phase was detected in the sample Pb1−x(La1/2K1/2)xTiO3 for x=1.0. The lattice parameters of Pb1−x(La1/2K1/2)xTiO3 (x=0.1, 0.2, 0.3, 0.4, 0.6, 0.8) were determined by high temperature XRD, and their intrinsic thermal expansion coefficients were obtained in the temperature range from room temperature to 900℃. In the temperature range from room temperature to the Curie point, the compound Pb0.8(La1/2K1/2)0.2TiO3 has a low intrinsic thermal expansion coefficient (−7.1×10−6℃−1), which is the value closest to zero in all the investigated compounds. The (ferroelectric) Curie temperatures of Pb1−x(La1/2K1/2)xTiO3 could be predicted from our experimental results for 0≤x≤0.4.