The swelling behavior of a surface-tethered poly(N-isopropylacrylamide) (polyNIPAAm) network in D2O was characterized with neutron reflection and compared to the demixing behavior of linear poly(NIPAAm) in solution. The surface-tethered network was fabricated by cross-linking a 250 A thick film of a copolymer comprised of NIPAAm and 3 mol % methacroylbenzophenone. As the temperature was varied between 15 and 29 degrees C, the thickness of the poly(NIPAAm) network in aqueous solution decreased gradually from I 100 to 750 A. In this regime, the network lay entirely in the single phase region of the phase diagram for linear poly(NIPAAm). At approximately 30 degrees C, the network entered the two-phase region of the phase diagram and collapsed along the tie line to a thickness of 339 A. Above 30 degrees C, the thickness of the network adjusted to the binodal curve of the phase diagram. The agreement between the swelling discontinuity in the surface-tethered network and the two-phase region of uncross-linked poly(NIPAAm) suggests that confinement does not alter the miscibility gap of poly(NIPAAm). With the use of a temperature, T, and polymer volume fraction, q5, Flory interaction parameter, v(TO), that is tabulated from the demixing data of the linear poly(NIPAAm), the swelling behavior of the surface-tethered network could be modeled using the mean-field Flory-Rehner theory modified for uniaxial swelling.