This thesis focuses on the utilisation of mid-infrared techniques in technologicalatmospheric pressure, non-thermal plasma (NTP) diagnostics. Two mid-infraredtechniques were demonstrated in this work namely laser absorption and Fouriertransform infrared (FTIR) spectroscopy. The performance of external-cavityquantum cascade laser (EC-QCL), a relatively new laser type with broad tuningcapability was also demonstrated as potential diagnostics tool for technologicalNTP applications. A dual plate dielectric barrier discharge (DBD) and a packed-bedNTP reactor were designed and fabricated to perform plasma process.Quantitative analysis of the laser absorption and FTIR spectroscopy techniquesfor gas detection were validated by using standard gas samples. Real-time COmonitoring by means of in-situ laser absorption spectroscopy measurements wereperformed for gas phase diagnostics in the decomposition of TEOS by means ofplasma-enhanced chemical vapour deposition (PE-CVD) and in CO2 reforming ofCH4 by means of NTP. In-line FTIR measurements simultaneously recorded thegas spectrum at the exhaust of the plasma reactors. Information from bothmeasurements was found to provide useful information on the plasma processesand chemistry for the NTP applications. Finally, wavelength stability and linearityperformance of a broad tuning range EC-QCL were evaluated by using the Allanvariance technique. (LOD) at SNR = 1 was estimated to be ~ 2 ppm, achievedunder atmospheric pressure, at the room temperature, and a path length of 41 cmfor NO detection produced from the decomposition of dichloromethane (DCM) bymeans of NTP.