Abstract Gas phase benzonitrile, acetone, 1-2 propanediol, fluorobenzene, and anisole molecules are produced in a cell at a temperature of 8 K, and detected via Fourier transform microwave spectroscopy (FTMW). Helium buffer gas is used to cool the molecules originating from a high flux room temperature beam. This general, continuous source of cold molecules offers comparable spectral resolution to existing seeded pulsed supersonic beam/FTMW spectroscopy experiments but with higher number sensitivity. It is also an attractive tool for quantitative studies of cold molecule–helium and molecule–molecule elastic and inelastic collisions. Preliminary data on helium–molecule low temperature rotational and vibrational relaxation cross-sections are presented. Applications of the technique as a sensitive broad spectrum mixture analyser and a high resolution slow-beam spectrometer are discussed. Keywords: molecular spectroscopybuffer gas coolingFourier transform microwave spectroscopy Notes Notes 1. The helium density is estimated from the known helium flow rate, temperature, and cell geometry; direct measurement of helium densities in similar geometries and flow regimes have confirmed that this calculation is accurate to within 30% Citation44. 2. This is only true in the ‘high temperature limit’, where the spectrometer central frequency ħω < k B T; below this temperature to noise scales as F = NT −3/2 R 1/2. For the 12–18 GHz band used here, the high temperature limit is applicable for any T ≳ 1 K. 3. In both our work and many supersonic beam experiments the total molecule number N is poorly quantified. Our steady state density is estimated at n = 1012, and total number at N = 1015. A typical ‘bright’ seeded supersonic source has a total N = 1 × 1015 molecules pulse−1, spread over a 500 µs pulse and moving at ≈1000 m s−1. Taking Citation18 as a typical example, the molecules are interrogated as they cross a microwave beam with a width of about 10 cm; at most 10% of the molecules are in the spectrometer beam at any given time.