We considered that the increase in smectic layer spacing when a bulky group is present could be caused by an increase in the orientational and/or translational order parameters. If we consider the change in layer spacing to be a consequence of size-exclusion effects similar to that observed in the solid state (Figure ), then we would expect no change in the orientational order parameters. We used the well-known method to determine the orientational order parameters of 9 and 8OCB from the WAXS data, and they are presented in Figure . The comparison of orientational order parameters in the nematic phase is complicated by the large difference in the temperature range of the nematic phase (ΔT8OCB >> ΔT9), however immediately prior to the SmA-N transition we find values of <P2> and of <P4> to be effectively the same for both materials. In the SmA phase we observe that at a given reduced temperature (TSmA-N / T) both materials have comparable values of <P2> and <P4>. The presence of a cyclic terminal group does not appear to affect the orientational order of the resulting mesophase, only the degree of positional order via segregation of the bulky cyclic terminal units. This effect can be considered as a steric pressure that results from the terminal group at the layer interface, reducing the interpenetration of aliphatic chains in adjacent layers and therefore giving increased layer spacing.