Triblock copolymers of poly(N-isopropylacrylamide)-block-poly(N,N-dimethylacrylamide)-block-poly(N-isopropylacrylamide) were synthesised via RAFT polymerisation using a symmetrical bistrithiocarbonate. Keeping the block length of the permanently hydrophilic middle block constant, the length of the poly(N-isopropylacrylamide) block was varied broadly. The thermoresponsive aggregation of the polymers in water was studied by 1H NMR, turbidimetry, and dynamic light scattering. The complex aggregation behaviour was block length dependent and occurred under kinetic control. Importantly, different information on the hydrophilic-hydrophobic transition of the poly(N-isopropylacrylamide) block was obtained using the various analytical methods and could not be directly correlated.