A parallel-plate transmission line has been developed for wavelengths between 8 and 11 cm and has been used for an examination of the properties of artificial dielectrics under conditions which approximate closely to those existing in free space. An experimental method by which the effective electrical length of dielectric samples can be determined has been shown to give reliable results. Particular attention has been paid to the occurrence of dispersion and a distinction is drawn between two forms of this. The first arises from the resonance of the elements from which the dielectric is constructed and is called “dipolar dispersion”because of the similarity to the corresponding behaviour of solid dielectrics. The second depends primarily on the spacings between the conducting elements and is referred to as “cavity dispersion. ” Experimental results are given for both types of dispersion.