Catalytic activity for ethylene dimerization is studied in four palladium-exchanged zeolites, PdH-clinoptilolite, PdH-mordenite, PdH-ferrierite, and PdHK-SUZ-4, with different channel systems. The formation and adsorbate interactions of NJ) are also investigated for a better understanding of the role of Pd(I) on the catalytic reaction in these zeolites using electron spin resonance (ESR) spectroscopy. Thermal and hydrogen reduction of Pd(II) in clinoptilolite, mordenite, ferrierite, and SUZ-4 produce isolated Pd(I) with somewhat different ESR parameters. The interaction of Pd(I) with various sizes of adsorbates shows some similarities in PdH-mordenite and PdH-ferrierite, indicating that Pd(I) is located in the main 12-ring and 10-ring channels of mordenite and ferrierite, respectively, where all the Pd(I) can coordinate with methanol and pyridine. However, SUZ-4 shows a noticeable difference in the way Pd(I) interacts with several adsorbates such as ammonia, methanol, and ethylene. These adsorbates reduce Pd(II) to Pd(I) at 298 K, leading to the formation of two Pd(I) ions situated at two different sites of SUZ-4. One is the same Pd(I) ion site formed by thermal reduction but it appears at higher intensity upon adsorption. The other Pd(I) is an isolated Pd(I) produced only after prolonged annealing with adsorbates. Our ESR and catalytic results show that Pd(I) is active for ethylene dimerization in these four channel-type zeolites. This activity is dependent on the location and accessibility of Pd(I) and the reaction temperature. All catalysts deactivate due to the further reduction of Pd(I) by ethylene and butene. Analysis of the composition of butene products indicates that PdH-clinoptilolite and PdHK-SUZ-4 reach a thermal equilibrium distribution faster than do PdH-mordenite and PdH-ferrierite.