Thin-film heat-transfer gauges are used in impulse hypersonic facilities to measure heat transfer rates to models immersed in the flow. These gauges and the associated instrumentation can be designed to have response times of the order of microseconds. Fluctuations in heat transfer inferred from such gauges are spectrally analysed in hypersonic laminar, transitional and turbulent boundary layers on a model of the BOLT-II flight geometry in the T4 Stalker Tube. The signal-to-noise ratio was reduced as much as possible because the process of inferring heat transfer from surface temperature signals is very sensitive to high-frequency noise. The spectral energy at higher frequencies increases as the layer progresses from laminar, through transition to fully turbulent. In a fully turbulent layer, the spectral power is found to vary with frequency, f, at \(f^{-0.7}\) between 20 and 150 kHz.