Block copolymers poly(N-isopropylacrylamide)-block-poly(ethylene oxide), PNIPA-b-PEO, were synthesized by free radical polymerization using macroazoinitiators bearing PEO chains with two different chain lengths (M w = 550 or 1900 g/mol). The molar mass of the PNIPA block varied from 0.4 × 105 to 7.3 × 105 g/mol. The cloud points of the aqueous copolymer solutions shifted to slightly higher temperatures only with the samples having the longer PEO block. Above the lower critical solution temperature (LCST) of PNIPA the block copolymers formed aggregates with a spherical core−shell structure sterically stabilized by a PEO shell. The formation and especially the shape of the aggregates were influenced by the length of the PNIPA block, the molar ratio of the repeating units of PNIPA and PEO, and the polymer concentration. A fluorescent probe 4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran, 4HP, was localized inside the polymer differently depending on the method of sample preparation. The microviscosity in the surroundings of the probe increased with the length of the PNIPA block. However, depending on the sample preparation method, the length of the PEO block was also observed to have an effect on the polarity of the environment around the probe. In general, increasing the block length of PEO affected critically the size and shape of the aggregates as well as the mass distribution within them above the LCST, owing to the enhanced solubilizing effect of PEO on the collapsing PNIPA and the improved steric stabilization of the aggregates induced by a PEO shell.