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.