In the present work the e4ects of cation ordering on dielectric properties were isolated by investigating three polymorphs of Ca(Ca 1/ 3 Nb 2/ 3 )O 3 that feature di4erent arrangements of the Ca 2ุ and Nb 5ุ cations on the B-sites of the perovskite structure. Dielectric measurements at frequencies above 1 GHz revealed a systematic dependence of the properties on the type of cation ordering. In particular, the structure with 2:1 ordering exhibited a lower dielectric constant and a signi5cantly more negative temperature coe7cient of resonance frequency than the structures with 1:1 and newly described k โซุโฌ 1 4 [1 1 1]* c ordering. Rietveld re5nements of structural models for the three Ca 4 Nb 2 O 9 polymorphs were conducted using X-ray and neutron powder di4raction data to elucidate structural details that could be correlated with the changes in dielectic properties. In all three polymorphs the cation ordering was combined with the same b ุ b ุ c ุ octahedral tilt system, and the structural re5nements yielded similar magnitudes of the tilting angles. The most signi5cant crystal-chemical di4erence between the three polymorphs was in the coordination environment of Nb 5ุ . Analysis of the re5ned bond distances indicated increasing average distortion of the Nb nearest-neighbor environment in going from the 1:1 to the k โซุโฌ 1 4 [1 1 1]* c to the 2:1 ordered structure. The increased fraction of strongly compressed Nb+O bonds in the 2:1 structure associated with the large distortion was correlated with the decrease in dielectic constant and more negative value of temperature coe7cient of the resonant frequency obtained for this polymorph. Raman spectra obtained for the three polymorphs exhibited di4erences that were consistent with the observed structural chemistry.