The present invention relates to a method for quantifying the composition of a product, including: irradiating the product with a radiation source in the near infrared range; receiving radiation which is reflected by or transmitted through the product; providing an output signal corresponding to the intensity of the radiation received at a number of different wavelengths; and determining whether or not the product lies within predetermined integrity criteria on the basis of the output signal using a mathematical method. The moving product contains a solution or homogeneous dispersion and the content of at least one substance contained in the dispersion or solution is quantitatively determined on the basis of the output signal. The invention also relates to a device for carrying out this method.