CsPbI ₃ is a promising material for optoelectronics owing to its thermal robustness and favorable bandgap. However, its fabrication is challenging because its photoactive phase is thermodynamically unstable at room temperature. Adding dimethylammonium (DMA) alleviates this instability and is currently understood to result in the formation of DMA _x Cs _1-x PbI ₃ perovskite solid solutions. Here, we use NMR of the ¹³³ Cs and ¹³ C local structural probes to show that these solid solutions are not thermodynamically stable, and their synthesis under thermodynamic control leads to a segregated mixture of yellow one-dimensional DMAPbI ₃ phase and δ-CsPbI ₃ . We show that mixed-cation DMA _x Cs _1-x PbI ₃ perovskite phases only form when they are kinetically trapped by rapid antisolvent-induced crystallization. We explore the energetics of DMA incorporation into CsPbI ₃ using first-principles calculations and molecular dynamics simulations and find that this process is energetically unfavorable. Our results provide a complete atomic-level picture of the mechanism of DMA-induced stabilization of the black perovskite phase of CsPbI ₃ and shed new light on this deceptively simple material.