The objective of this work was to study with near infrared spectrometry (NIRS) the degree of mixing of poorly miscible binary mixtures of carbamazepine (CBZ) and alpha-lactose monohydrate (LMH). CBZ was cohesive and the particle size difference between CBZ (0.3microm) and LMH (75microm) was substantial. Mixed batches were measured directly in the mixing container with a fiber-optic probe. The spectral data were filtered by applying a novel automated selection technique during the NIRS measurement. The data analyses were performed with partial least squares modeling. Reference measurements were carried out with ultraviolet spectrophotometry. The results describe the degree of homogeneity at various depths. Some of the mixtures densified when the mixing speed increased. The densification of the batches was a source of error because it caused changes in the measuring geometry. The automated selection technique for the spectra reduced this problem. NIRS detected differences in the mixing degrees of mixtures and the method is suitable for mixing studies. However, the difference in the particle size of the materials and the densification caused problems to the measuring geometry. NIRS can be used at line, but the method requires accurate operation and method developing before it is useable.