Careful measures are taken in the oil and gas industry to avoid the formation of gas hydrate plugs. However, the risk of inhibitor failure, unexpected process changes, and more frequent transport of underinhibited well streams calls for methods to monitor gas hydrate formation. In gas-dominated systems, hydrate plug formation is believed to be initiated by formation of hydrate layers on the pipeline wall. The permittivity spectra of gas hydrates differ from those of water, oil, and gas. Hence, a potential approach to detect early stages of plug formation in gas-dominated systems (i.e., before hydrates become a threat to flow assurance) is to monitor the permittivity of the flow close to the pipeline wall. Such technology is also valuable for lab-scale studies on plug formation mechanisms in gas-dominated systems. This paper presents broad-band permittivity measurements on gas hydrate layers using open-ended coaxial probes. Experiments are conducted both at atmospheric conditions, using tetrahydrofuran as the hydrate former, and at high pressures, using a mix of methane and propane as the hydrate former. Measurements on ice layers are included for comparison. Formation of hydrate layers thinner than 1 mm is shown to be easily detected.