Near infrared spectra were recorded during a polymorph conversion of P1 into P2. The spectra were measured on-line in the reaction vessel, which was held at a constant temperature. The dissimilarity criterion used in the orthogonal projection approach (OPA) was evaluated to follow the reaction progress and to detect the end point of the conversion process on-line. The soft independent modelling of class analogy approach (SIMCA) was also investigated for the detection of the start and the end point of the reaction.