The physical properties of active pharmaceutical ingredients (APIs) have to meet stringent specifications such as a defined particle size distribution and polymorphic form. These physical properties are often controlled in the final API crystallization step by the application of sophisticated process analyzers so called process analytical technologies (PAT) tools. Here twelve different pharmaceutical compounds are analyzed by seven different online analytical measurement techniques (ATR-MIR, Raman-, NIR and UV-Vis spectroscopy, turbidity, ultrasound and optical reflectance measurement) in lab and pilot plant scales. The techniques are used for metastable zone width determination, solute concentration measurement and to provide information on the solid phase (e.g. polymorph content) during crystallizations of APIs. The results demonstrate that the techniques differ strongly in sensitivity e.g. to detect the presence of particles or to detect concentration changes. It is found that various effects can have an influence on the direct transferability of different calibration models, which prevent or limit a direct transferability.