Narrowly distributed ultralong linear poly(N-isopropylacrylamide) (PNIPAM) (M-w = 2.63 x 10(7) g/mol and M-w/M-n < 1.1) was successfully prepared by a combination of fractionation and filtration. Using this PNIPAM sample, we have studied the conformation change of individual PNIPAM chains in different methanol/water mixtures at 20 C. Both water and methanol are good solvents for PNIPAM, but not a proper mixture of them. The PNIPAM chain collapsed sharply from a coil to a stable globule when the methanol content (x(methanol)) reached similar to 17 mol %. In the range 17-40 mol %, the PNIPAM chain remained in the globule state. Further addition of methanol finally led to a globule-to-coil transition at x(methanol) similar to 50 mol %. The reentrant coil-to-globule-to-coil transition could be attributed to the formation of different water/methanol complexes, presumably, (H2O)(m)(CH3OH)(n), which are poor solvents for PNIPAM. We have,for the first time, revealed that the conformation change of individual polymer chains can be used as an indicator to probe the complexation, similar to an indicator used in titration. Our results suggest that when x(methanol) < 17 mol %, the water/methanol molar ratio (m/n) is similar to5, while when X-methanol > 50 mol %, m/n becomes 1. In the range of similar to 17 mol % < X-methanol < 50 mol %, different intermediates with 1 less than or equal to m/n less than or equal to 5 coexist. The speculated structures of these complexes could be related to a water pentamer.