(i) The high speed of Fourier Transform spectrometry can give Quantitative spectra at a sufficiently high rate to be useful for on-line analysis in the wavelength 2.5-15 μm. By designing an instrument for on-line use, a simplified interferometric spectrometer has been developed at a cost that is viable for on-line use. The method may be applied for analysis of gases, liquids and certain types of solids. (ii) A single-component infrared analyser has been developed, which achieves absorption of the beam by the liquid sample using an attenuated total reflection crystal. Although the optics and wavelength selection method (interference filters) are conventional, the infrared measuring head has been built as a miniature sealed arrangement that can be submerged in the process stream if required. It has been found suitable for measurements on a range of samples including aqueous solutions.