Most studies of coil-to-globule transitions of macromolecules have dealt exclusively with the standard polystyrene-cyclohexane system. In this work we aim to extend such investigations to halogenated polystyrenes, namely poly(4-chlorostyrene)(PPCS), poly(2-chlorostyrene)(POCS) and poly(chlorostyrene)(PCS),a mixture of 2- and 4-isomers. Changes in R-h Of these polymers with temperature in their respective theta solvents were investigated by dynamic light scattering. The hydrodynamic size of PPCS in a-propyl benzene(M-W : 2.25 x 10(6) g mol(-1), M-W/M-n 1.16) decreases to 75% of that in the unperturbed state at 22.2 degrees C. POCS (M-W : 10.32 x 10(6) and M-W : 4.30 X 10(6), M-W/M-n : 1.10) in 2-butanone exhibited LCST behaviour and a Limited chain collapse was observed with increasing temperature above theta point. The parameters alpha(h) = R-h/R-h(theta) for POCS with two different molecular weights, (10.32 M and 4.30 M) were found to be 0.67 and 0.77, respectively at 37.0 degrees C. A smooth and continuous contraction was also observed for the mixture of 2- and 4-isomers, PCS (M-W : 2.32 x 10(6) g mol(-1), M-W/M-n : 1.06).