A method for quantitative in-situ measurements of the chemical content of powder mixtures is presented. By means of near infrared spectroscopy (NIR) and a fiber-optic diffuse reflection probe inserted into the powder in the blender, determinations of the powder mixture content can be made during processing. A simplified methodology for multivariate calibration is used to determine the powder content from the NIR spectra. With this methodology, calibration samples are prepared and analyzed off-line. The presented method can be used to monitor the content and the homogeneity of powder mixtures in real time and thus to assess the performance of powder handling operations, such as powder mixing, during development, scale-up and production. In this paper, an Fourier transform (FF)-NIR spectrometer was used to monitor the mixing of a segregating powder mixture, consisting of a coarse (d > 300 mum) and a fine powder (d < 200 mum), in a convective mixer (Nauta mixer). Both a small laboratory-scale and a large production-scale mixer were used. It is demonstrated that by high-speed sampling, both the average content and the distribution of the mixture content can be assessed during processing. The experimental content vs. process time trajectories are in good agreement with a theoretical simulation model for blending in Nauta mixers.