Fourier transform infrared spectroscopy is a powerful tool for measuring the spectral radiation from flames and industrial processes. The effects on the spectrum from flame fluctuations occurring naturally during the measurement are analyzed briefly and illustrated by experiments and simulations. Results from applications on an oil flame, a pulverized coal flame, and a full- scale waste incinerator are described. The experiments show that important information about the composition and temperature of gas and solid temperatures might be extracted from the emission spectra. The practical and technical limitations of the technique and possible future applications are discussed.