Spherical microgels were prepared by precipitation copolymerization of N-vinylcaprolactam and sodium acrylate [P(VCL-co-NaA)] in water at 60 degreesC. Because it is a thermally sensitive polymer, the increase of temperature in the range 25-40 degreesC leads to a continuous shrinking of the poly(N-vinylcaprolactum) (PVCL) chain. The copolymerization of a few molar percent of NaA into a PVCL chain increases the extent of its swelling and shifts the temperature at which it shrinks to a slightly higher level. Our results revealed that, in the shrinking process, calcium ions (Ca2+) could induce a profound thermoreversible complexation of the P(VCL-co-NaA) microgels at a critical temperature (T-c) which was nearly independent of the NaA content. However, both the rate and degree of the complexation at T-c increased with the NaA content. A comparison of linear P(VCL-co-NaA) chains and spherical P(VCL-co-NaA) microgels in the complexation is presented.