Abstract: The properties of a liquid hydrocarbon blend are determined from the NIR (Near Infra Red) spectrum of the components of the blend. This is accomplished by: (a) determining with an IR spectrometer, the absorbance of the components of the blend at a certain number of frequencies in the spectral range 16667 to 3840 cm.sup.-1 starting from a defined base line, (b) determining for each component and each property, a spectral mixture index (SMI) by applying a correlation between the SMI and the absorbance values, and then (c) calculating the property (J) to be determined by applying a linear expression, J=f.sub.a SMI.sub.a.sup.J +f.sub.s SMI.sub.b.sup.J +f.sub.c SMI.sub.c.sup.J + . . . f.sub.o SMI.sub.o.sup.J where SMI.sup.J.sub.(a, b, c . . . ) is the Spectral Mixture Index for property J for each component (a, b, c . . . ) and f.sub.(a, b, c . . . ) is the fraction by volume of that component in the final blend.

Abstract: The properties and yield of a product from a hydrocarbon conversion or separation process may be determined from the NIR (Near Infrared) spectrum of the feedstock. This is accomplished by: (a) determining with an IR spectrometer the absorbance of the feedstock at a certain number of frequencies in the spectral range 16667 to 3840 cm.sup.-1 starting from a defined base line, and then (b) determining the property or yield by applying a correlation between the property or yield and the absorbance values. The correlation is determined experimentally by multivariate regression and is dependent upon the type of spectrometer employed, the property or yield to be determined, and the frequencies used.