Abstract: A method of determining or predicting a value P.sub.x of a property (e.g. octane number) of a material X or a property of a product of a process from the material or yield of the process for example a blending, separation, or chemical (e.g. polymerization) process, which method comprises measuring the absorption D.sub.ix of the material at more than one wavelength in the region 600-2600 nm, comparing the the signals indicative of the absorptions or a derivative thereof with signals indicative of absorptions D.sub.im or derivatives thereof at the same wavelength for a number of standards S in a bank for which the property or yield P is known, and choosing from the bank at least one standard S.sub.m with property P.sub.m the standard having the smallest average value of the absolute difference at each wavelength i between the signal for the material and the signal for the standard S.sub.m to obtain P.sub.x, with averaging of the properties or yields P.sub.m when more than one standard S.sub.m is chosen.