The present study focuses on the swelling behavior of copolymer microgels of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc) containing different concentrations of acrylic acid. The incorporation of the comonomer is investigated by Fourier transform infrared (FT-IR) spectroscopy and an incorporation threshold value for the comonomer is found, if the "classical" Pelton synthesis approach is used. The swelling of these microgels is analyzed by photon correlation spectroscopy (PCS) and fluorescence spectroscopy, using pyrene as a fluorescent probe. The deswelling of copolymer microgels with 10mol% and 15mol% nominal acrylic acid content can not be detected by PCS. However, in fluorescence measurements the PNIPAM phase transition is still observable. The reason for this difference is a non-uniform distribution of acrylic acid in the copolymer microgels, which makes them promising candidates for drug-delivery and - targeting applications. (C) 2017 Elsevier Ltd. All rights reserved.