From DSC trace, it can be concluded that the 1st peak is due to desolvation or dehydration, the 2nd due to melting and followed by degradation. The melt peak can be contaminated by degradation as two events occurred and overlapped in the same or similar temperature range.

Jp Wu , thank you so much for your answer and support. Could this dehydration be due to the evaporation of the residual/adsorbed moisture on the compound?

It would be helpful to know the material composition

It looked like a bonded water not surface moisture from your DSC and inotial weight loss in TGA. Could you just show the both below 150deg.

Jp Wu here is the attached file for TGA (red) and DSC (black) at temperature below 150deg. Hope it helps

Prof Gyorgy Banhegyi it is an organic compd which we are looking at to study its thermal stability. Generalised structure is shown below where R groups are various ring substituents.

I presume you made the TGA and DSC curves were measured in an inert atmosphere. Are the R groups polar? I mean, can they attract moisture? Did you try to compare with previously thoroughly dried samples? In this case you could confirm whether the first peak is really due to water or not. Do you have access to FTIR? It could also help to identify (or at least compare) the compounds before and after decomposition. It is also interesting to see that both endothermal peaks above 300 degC are fairly sharp. Decomposition endotherms in organic materials are usually broader (as is the DTG peak). Is it not possible that the material loses some functional groups and it recrystallizes and melts again? There is a slight indication of that in the fact that between the two melting peaks there seems to be an exotherm (recrystallization). If you have access to pyrolysis GC-MS you could determine the evolved gases at various decomposition temperatures.

Prof Gyorgy Banhegyi thank you so much. Yes, the R groups are heterocyclic rings, containing NH and carbonyls. I took an FTIR and the 3000-3400 cm-1 range shows somewhat broadened peak (indicating residual moisture) where NH stetch occurs. The compounds after synthesis were stored in P2O5 dessicator (vaccum) for 24h and then under CaCl2 dessicator for 2-3days. The expt were carried out under N2 conditions. As you have said, the double bond (exocyclic) might break after 1st melt and again recrystallize as in DSC peaks. But, doesnt it seem that the 2nd sharp peak in DSc at around 323 deg correspond to decomposition as inTGA there is nearly 32%wt lossof the compd at that temp?

Well, you could do two things. First, calculate the mass of the (possibly leaving) side groups related to the total molecular mass to see if the 32% loss corresponds to the expected loss. Surely, even if a new crystal appears, it also begins to decompose at a temperature close to the second melting. You may try to go up to, say 300 oC, and from that point on you could slow down the heating rate. That will give a higher resolution. Another thing you may try is to go to 314 oC, watch the weight and enthalpy changes for e.g. 5 minutes, then cool back the system. If it is a new crystal, you can heat it again (and you can make FTIR or XRD). If it is a new compound, it will show only one melting point (followed by decomposition).

Please zoom in the y axis for better observation of signals.

Such two sharp peaks at high temperatures due to decomposition was observed before. Multisampling can help identify what are those two sharp peaks, i.e. repeatable two peaks suggesting true melts.