Statistical copolymers of N-isopropyl acrylamide (NIPAM) and N,N-diethyl acrylamide (DEAAM) show a pronounced synergistic depression in their cloud points, though both homopolymers phase separate at significantly higher temperatures close to 30 degrees C (e.g., Polymer 2009, 50, 519). While phase separation occurs at 20 degrees C for the statistical copolymers, the influence of the monomeric sequential arrangement along the backbone was not addressed so far. Thus, we report on the thermosensitive properties of a diblock copolymer PDEAAM-b-PNIPAM and compare it to the homopolymers, mixtures thereof, and to the statistical copolymer of the same molecular weight. These polymers were prepared by controlled radical polymerization, namely Reversible Addition-Fragmentation Chain Transfer (RAFT). Their solution behavior was mainly studied by infrared spectroscopy (IR) of the amide I' band and by turbidimetry. IR spectroscopy sees a decreasing hydration with heating even below the cloud point for all polymers. This results finally in phase separation, which induces further spectral changes. Rather unexpectedly, the diblock copolymer shows phase separation at temperatures close to the homopolymers, well above the cloud points of the homopolymer mixtures. In turn, the transition temperature of the homopolymer mixture is reduced compared to its homopolymers, which indicates intermolecular attraction between both partners. This behavior can be explained by taking the block length dependencies of the respective cloud points into account and assuming a rather independent phase behavior of each short block (within the copolymer). Then, the increased inherent cloud point of each "half-length" block (compared to the homopolymers) has a stronger effect than the aggregating tendency inherited by the connectivity of the comonomer units. As a result, IR spectroscopy reveals almost ideal behavior of the diblock copolymer, which can be comprehended as an ideal mixture of the homopolymers, each one contributing to the overall signal by its concentration. Finally, H-1 NMR suggests that intermediate aggregation (as seen by light scattering) is not induced by segregation of just one block, but rather by partial and weak complexation between the two components within the diblock copolymer.