Combined experimental and first-principles studies have been conducted to obtain a fundamental understanding of the effect of A-site chemical doping on the electronic structure and ferroelectric behavior of Pb(Ca,Ba)TiO 3 , Pb(Sr,Ba)TiO 3 , and Pb(Sr,Ca)TiO 3 perovskite complex solid solutions. Rietveld refinement of powder X-ray diffraction data shows that the crystal structure of all the three compounds is distorted from the ideal cubic perovskite structure. At the nanoscale, piezoresponse force microscopy (PFM) studies show low-performance ferroelectric properties of Pb(Sr,Ca)TiO 3 thin films when compared to Pb(Ca,Ba)TiO 3 and Pb(Sr,Ba)TiO 3 films. Theoretical analysis of the electronic band structure performed on the basis of density functional theory (DFT) allows to elucidate the origin of the different ferroelectric behaviors observed in Pb(Ca,Ba)TiO 3 , Pb(Sr,Ba)TiO 3 , and Pb(Sr,Ca)TiO 3 thin films. DFT-based computational calculations reveal that there is a strong correlation between the effects of Ti 3d non-bonding orbitals (responsible for p TieO bonding) and the ferroelectric polarization behavior of A(A 0 A 00)BO 3 complex perovskite solid solutions. In our study, very low Ti 3d xy, d xz , and d yz non-bonding electronic density state contributions were observed and the presence of mainly ionic CaeO and SreO bonds. These effects are the reason for the unusually weak polarization, low tetragonality, and poor ferroelectricity of Pb(Sr,Ca)TiO 3 thin films. This is in contrast to the observed behaviors of Pb(Ca,Ba)TiO 3 and Pb(Sr,Ba)TiO 3 thin films. However, our first-principles calculations agree well with the PFM-based experimental results obtained in the nanometer scale.