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In continuation of our earlier work on smectogenic compounds exhibiting large bilayer spacings, we report in this paper a comparative study of several compounds with lateral methyl or methoxy substituent. Some of the compounds do not have any strongly polar end groups while others have a cyano end group. The dielectric (both static and relaxation studies up to 13 M E ) and X-ray investigations on these compounds enable us to elucidate the physical mechanisms responsible for the observed differences in the properties of the compounds. We also discuss the possible origin of the often observed reduction of the activation energy corresponding to the c,, -relaxation in the smectic A phase compared to that in the nematic phase.