Methylammonium (MA)- and formamidinium (FA)-based organic inorganic lead halide perovskites provide outstanding performance as photovoltaic materials, due to their versatility of fabrication and their power conversion efficiencies reaching over 22%. The proposition of guanidinium (GUA)-doped perovskite materials generated considerable interest due to their potential to increase carrier lifetimes and open-circuit voltages as compared to pure MAPbI(3). However, simple size considerations based on the Goldschmidt tolerance factor suggest that guanidinium is too big to completely replace methylammonium as an A cation in the APbI(3) perovskite lattice, and its effect was thus ascribed to passivation of surface trap states at grain boundaries. As guanidinium was not thought to incorporate into the MAPbI(3) lattice, interest waned since it appeared unlikely that it could be used to modify the intrinsic perovskite properties. Here, using solid-state NMR, we provide for the first time atomic-level evidence that GUA is directly incorporated into the MAPbI(3) and FAPbI(3) lattices, forming pure GUA(x)MA(1-x)PbI(3) or GUA(x)FAI(1-x)PbI(3) phases, and that it reorients on the picosecond time scale within the perovskite lattice, which explains its superior charge carrier stabilization capacity. Our findings establish a fundamental link between charge carrier lifetimes observed in photovoltaic perovskites and the A cation structure in ABX(3)-type metal halide perovskites.