The thermolysis routes of two isostructural metal-organic framework compounds (Zn-based ZIF-8 and Co-based ZIF-67) are investigated based on temperature-dependent and time-dependent in situ Fourier transform infrared (FTIR) spectroscopy and in situ X-ray diffraction data, as well as thermogravimetric-differential scanning calorimetry (TG-DSC) analyses and density functional theory (DFT) calculations. These data highlight thermolysis effects on different vibrations and dissociations within specific atomic moieties. The coordination differences between Zn-N and Co-N lead to the distinct thermolysis routes of ZIF-8 and ZIF-67. ZIF-8 is easily deformed during heating while decomposes at a higher temperature due to the saturated Zn-N coordination. ZIF-67, however, does not deform during heating due to the stronger Co-N bonds, but easily reacts with oxygen due to the unsaturated Co-N bonds. Our results demonstrate that in situ FTIR paired with in situ XRD is a powerful technique for MOF thermolysis investigation, and we suggest that the thermolysis mechanisms of MOFs may be unveiled by investigating a series of MOFs having different coordination types using in situ characterisation methods.