Metal working fluid (MWF) emulsions are utilized as coolants and lubricants in machining processes like turning or drilling. During their operation life time cycle, MWFs change their properties due to impacting stresses which may influence the machining and tool performance. A frequent refreshing or renewal of MWFs in machining process is thus necessary. This investigation discusses measurement techniques of MWF emulsions to be used for MWF quality assessment and process monitoring. By means of optical spectroscopic measurement techniques (turbidimetry and laser diffraction), the evaluation of the temporal change of the wavelength exponent and the MWF emulsion droplet size is related to the MWF stability. The in-process monitoring of the MWFs in machining during several weeks of operation is shown. Thus, it will be demonstrated that optical spectroscopic measurement techniques may be applied to determine stability change of the emulsion system.