An appropriate thermal behavior of polymer powders is essential for successful application in Selective Laser Sintering (SLS) process. A narrow and not sufficient “Sintering-Window” is a primary hurdle for a lot of semi-crystalline Polymers in this connection. Thermo-analytical (DSC) measurements are used to understand the behavior of polymers during SLS-process and to reveal the difference between proper and less proper thermal properties. Commercial PA12-SLS powders are investigated in this sense and compared to a non-commercial one. Thus, the contribution summarizes some very basic polymer properties essential for successful SLS application and explains consequently at the same time why just very few polymers are applicable to SLS today.