The interchain contraction and association of polystyrene in semidilute solution near the Theta temperature are studies using NET. The results reveal that during a drop of temperature, polystyrene chains pass through different states: a smooth change of conformation at temperatures above the Theta temperature, an accelerated and association, aggregation of polymer chains, and then further continuous contraction below the Theta temperature. A pronounced hysteresis is observed during repeated heating. A viscosity study of the same solution was compared with the NET results. It indicates that NET can effectively be used to study conformational changes like contraction and association of polymer chains in a semidilute solution.