Phosphorus (P) originating from lubricant oil additives or biofuels is an emerging chemical poison in catalytic systems for automotive exhaust after-treatment. Here, we demonstrate that P-poisoning led to severe deactivation of small-pore Pd-SSZ-13 zeolites (with CHA framework) as passive NO_x adsorbers (PNA) and CO oxidation catalysts for cold-start exhaust purification applications. Deactivation mechanisms of P-poisoning were unraveled by comparatively examining the P-free and P-loaded Pd-SSZ-13 zeolites using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), temperature-programmed reduction by hydrogen (H₂-TPR), CO pulse adsorption, temperature-programmed desorption using NH₃ as a probe molecule (NH₃-TPD), ultraviolet/visible light (UV/vis) spectroscopy, and in situ diffuse relectance infrared Fourier transform spectroscopy (DRIFTS). The loss of isolated Pd sites─namely, [Pd(OH)]⁺ and Pd²⁺ (located in the eight- and six-membered rings of CHA framework, respectively)─was revealed to be largely responsible for the deactivation of Pd-SSZ-13 in passive NO_x adsorption and catalytic CO oxidation. In situ DRIFTS studies using NO or CO as a probe molecule suggest that [Pd(OH)]⁺ was more susceptible to P-poisoning than Pd²⁺. Specifically, P-poisoning led to a migration of [Pd(OH)]⁺ from cationic exchange sites to the zeolite surface, forming inactive metaphosphate (i.e., [Pd(OH)]⁺PO₃^–) and bulk PdO_x species at high temperatures. In contrast, P-poisoning of Pd²⁺ sites proceeded via a sequential transformation to [Pd(OH)]⁺ first, and then to [Pd(OH)]⁺PO₃^– and bulk PdO_x. This study provides a comprehensive mechanistic understanding on the deactivation of Pd-SSZ-13 by P-poisoning, and may guide the design of high-performance, phosphorus-resistant Pd-zeolite catalysts for cold-start exhaust after-treatment.