Oxygen-pretreated, palladium-exchanged silicoaluminophosphate SAPO-11, SAPO-5, and SAPO-8 are catalytically active for ethylene dimerization. The catalytic activity of the silicoaluminophosphates is shown to be due to Pd(I) species and is greatly dependent on the large channel size of the supporting materials which vary from 10-ring (SAPO-11) to 12-ring (SAPO-5) to 14-ring (SAPO-8) sizes. The selectivity for the formation of n-butenes is influenced by the reaction temperature and the channel size, showing that lower temperatures and larger channel diameters favor n-butene formation. Electron spin resonance studies show isolated palladium(I) species A (g(parallel to) = 2.963 and g(perpendicular to) = 2.141) and B (g(parallel to) = 2.678 and g(perpendicular to) = 2.078) after activation in SAPO-5 and SAPO-11. In SAPO-8 only species A’ (g(parallel to) = 2.91 and g(perpendicular to) = 2.12) can be observed. After ethylene adsorption in all materials, species G (g(parallel to) = 2.245 and gl = 2.003) can be seen, which transforms to species H (g(parallel to) = 2.421 and g(perpendicular to) = 2.033) and K (g(parallel to) = 2.641 and g perpendicular to = 2.076) during the reaction with simultaneous butene formation detected by gas chromatography. Electron spin echo modulation analysis allows the assignment of species G to a Pd(I)-C2D2 complex and species H to a Pd(I)-C4D8 complex. This work shows that in silicoaluminophosphates monovalent palladium cations coordinate ethylene and are catalytically active for ethylene dimerization.