An investigation of the various proper and improper dominant mode solutions that may exist on a stripline with an air gap above the conducting strip is reported. The modes are called “dominant,” even though they sometimes do not propagate down to zero frequency, because the strip current has the same form as that of the conventional stripline dominant mode. The improper modes, which are not well known in this context, do not exist on stripline with a homogeneous substrate but may exist when the substrate is made inhomogeneous (for example by the presence of an air gap). These modes, whose fields increase to infinity transversely, are present in addition to, and independently of, the proper dominant mode, which has bounded fields. Depending on the frequency and geometrical parameters, the improper modes may be either leaky modes, which are physical and have a complex propagation constant, or improper real modes, which are nonphysical. Numerical results are presented that describe the properties of both the proper and improper modes and how they change with frequency and air gap size.