A method of estimating the relative concentration of at least two components contained in a mixture of the components is disclosed. At least two mixtures are produced by combining the at least two components, each of the at least two mixtures having different concentrations of the at least two components. NIR mixture spectra are acquired from each of the at least two mixtures. The NIR component spectra and the NIR mixture spectra are input into a computer utilizing chemometrics software and the spectra are analyzed to produce a calibration model for each component and each of the mixture NIR spectra. NIR monitored spectra for a monitored mixture of the components having an unknown concentration of the components is acquired. The calibration models are applied to the NIR monitored spectra to thereby estimate the concentration of at least one of the components in the monitored mixture. The method can be applied to monitor fluid mixtures produced from one or multiple hydrocarbon reservoirs and surface facilities including pipelines and tanks.