The premise of the experiments described here is to study laminar-turbulent transition of hypersonic boundary layers (BL) on a flat plate in two different hypersonic facilities. The fundamental theoretical framework for investigations of compressible BL transition constitutes Mack’s linear stability theory (LST)[7]. He demonstrated that hypersonic BLs contain besides vorticity disturbances (first mode), multiple acoustical instability modes (Mack modes). The first of these Mack modes, the second mode, is the most dominant and most unstable disturbance mode in such planar hypersonic flat-plate BLs. The dominance of the second-mode instability was shown in extensive experimental studies for conical BLs and the results agree well with theoretical LST predictions (see review by Stetson [12] and references therein). However, experiments studying planar BLs (flat plate:[5],[14], hollow-cylinder:[13]) show that the “second-mode disturbances appear to play only a minor role in the transition process”. Low frequency disturbances seem to dominate the flow. The disturbances grow in a frequency band that is typical for the first mode and even in a range that LST predicts to be stable. A dominance of the second mode could not be observed. The issue of the planar-versus-conical BL anomaly is extensively discussed in [13], including comparisons with previous experiments, parameter effects like noise level, unit Reynolds number dependency etc. The instabilities of planar BL appear to be fundamentally different from the conical case. Several question were raised (eg different receptivity to freestream disturbance, prediction by LST etc.) and many aspects of this phenomenon remain unclear. Most of the stability experiments mentioned above were conducted using hot-wire anemometry. In scope of the above questions, there are several drawbacks to this technique for the determination of growth rates of instability waves: Besides the limited frequency response and mechanical strength of hot-wires, their downstream influence excludes simultaneous streamwise amplitude measurements. Non-intrusive techniques for simultaneous measurements using streamwise arrays would permit a more precise determination of growth rates. Therefore, surface mounted measurement techniques (here: commercial pressure sensors [9] and ALTP heat flux gauges [10]) with high spatial and temporal resolution are used in the present experiments. Instability waves are measured by a staggered array of these single-point sensors and spatial amplification rates are determined from quantitative amplitude spectra of fluctuations of surface pressure and heat flux, respectively. The spectra show two dominant disturbances, one growing in the low-frequency range for a certain unit Reynolds number regime, and a second present in the frequency range typical for the second-mode instability. Comparisons of the growth rates of the latter disturbance mode with linear stability theory computations exhibit a good agreement of amplification rates in the early stages of the transition process at least for the ALTP measurements.