Thermosensitive polymers such as poly(N-isopropylacrylamide) (PNIPAM) have a lower critical solution temperature in water in a temperature range between 32 and 35 degreesC. Micro- and macrogels of such polymers show a temperature-controlled reversible swelling behavior. Therefore, they are of great interest for a variety of applications (e.g., the controlled drug delivery and accessibility of enzymes). Up to now such gels were prepared by covalent cross-linking. A new and promising route for the preparation of microgels down to the nanometer scale consists of the formation of polyelectrolyte complex particles between ionically modified thermosensitive polymers. The formation, structure, and temperature behavior of complexes between PNIPAM with differing contents of anionic and cationic groups was studied by viscometry and static and dynamic light scattering. The level of aggregation of such complexes increases with increasing mixing ratio, while the polymer concentration has only a marginal effect. The complexes were highly swollen at 25 degreesC and collapsed in a temperature range up to 50 degreesC, where the swelling-deswelling process is completely reversible. Even complexes between copolymers of higher contents of ionic groups (up to about 30%) and complexes between copolymers and pure polyelectrolytes showed a pronounced temperature dependence of the structural density.