The widespread use of coplanar waveguide for millimetre wave integrated circuits has extended in recent years to frequencies well above 70 GHz and even as high as 200 GHz has been reported. At the higher frequencies, some problems with unwanted modes have been reported. This type of waveguide possesses an unwanted coplanar mode, which is fairly easy to suppress using air bridges. However it is also possible to couple energy from coplanar waveguide into unwanted substrate modes above a critical frequency. This paper discusses this critical frequency and gives examples of typical values and equations to enable the reader to calculate it for the general cases. This paper concludes with some of the practical consequences of this coupling.