Publisher Summary This chapter describes the characterization of polyketone (PK) polymer systems by thermal analysis techniques. Aliphatic polyketones based on carbon monoxide and ethylene are called PK copolymer, while the first commercialized grades based on carbon monoxide, ethylene, and a small amount of propylene are called PK terpolymer. The PK copolymer chains can crystallize under certain circumstances into different modifications. The chapter investigates the amorphous phase of PK terpolymer by dynamic mechanical analysis/differential scanning calorimetry (DMA/DSC). Two effects are noticed during the determination of the mechanical properties of PK terpolymers—the flexural modulus increases as a function of time during storage, and a weight increase of the test samples due to moisture uptake. A modulus value increase upon storage under ambient conditions is also reported for other semi-crystalline polymers.