The formation of associated defects (e.g.[Al _Ti-V _O]˙) upon acceptor doping is commonly seen as a reason for trapping of mobile vacancies in perovskite ionic conductors and electromechanical hardening in piezoelectric perovskites. In order to clarify the presence of associated defects in Al-doped (Na _1/2,Bi _1/2)TiO ₃(NBT-Al) and Al-substituted ((Na,K) _1/2Bi _1/2)TiO ₃-BiAlO ₃(NKBT-BA), we employ a combination of impedance spectroscopy, ²⁷Al NMR spectroscopy, and electronic structure calculations. Our results indicate that associated defects between and oxygen vacancies can only be found in case of low acceptor doping concentrations. This suggests a decreased driving force for defect association at high doping concentrations as the reason for the non-linear dependence between acceptor concentration and oxygen ionic conductivity for NBT-based ceramics. Furthermore, the combination of experimental and theoretical techniques provides clear evidence for the successive occupation of the B-site, the A-site, and finally the formation of a secondary phase with increasing Al ³⁺content. Altogether, these results call for a new evaluation of the interaction between aliovalent dopants and O ²⁻vacancies in acceptor-doped functional oxides, with implications for the design of ionic conductors as well as ferroelectric materials.