The thermal deblocking of masked isocyanate model substances was investigated by means of simultaneous thermal analysis and quadrupole mass spectrometry system (STA/QMS). The results were compared with additional measurements carried out by differential scanning calorimetry (DSC) and temperature dependent Fourier transform infrared spectroscopy (FTIR). The measured values which were determined as the deblocking temperature depend on the method of investigation. Thermogravimetric and mass spectrometric data depend on the volatility of the components and could differ from those observed by DSC and/or FTIR. Furthermore, the mechanism of deblocking is very complex. Secondary reactions obviously play an important role.