More often than not sodium perchlorate is used to facilitate crystallisation. To counterpoise large cations an equally large anion is required to promote closepacking. Although the perchlorate ion is only of medium size, it always seems to fill the void adapting its geometry accordingly. This is commonly interpreted as static disorder which is locked in at the time of crystal growth. However, this is not easily distinguishable from dynamic disorder, unless it's analysed as a function of temperature. Study of a macrocycle at room temperature [1] and at 130K [2], revealed that the disorder of the macrocycle is unaffected by cooling but that the perchlorate favoured one arrangement when cooled. The more rigid environment appeared to impose more structure on disordered perchlorate ions in the crystal. This suggested that the perchlorate be considered as structureless in solution and to assume [3] a tetrahedral shape in the solid state at a low temperature. An extensive search of the Cambridge Database revealed ordered and disordered perchlorates and it is not always easy to distinguish between the two instances. Investigation of the relationship of the free volume and the degree of disorder reveals that the larger the available space for the perchlorate ion, the higher the degree of disorder that results. A program [4] that calculates the volume of the molecule was then used to calculate the free available volume from the total cell volume. Comparisons of structures at room temperature and at low temperature demonstrate the relationship between the free volume available and the degree of disorder displayed by the perchlorate ion.