High-accuracy cloud-point temperatures for polystyrene-cyclohexane (PS-CH) and polystryrene-deuterated cyclohexane (PS-CH-d) solutions were measured over a broad range of molecular weights: 2.5 × 104 ≤ Mw ≤ 1.32 × 107. All polystyrene samples were characterized by low polydispersity indexes. Deuterium substitution in cyclohexane shifts the UCSTs to higher values and this isotope shift increases with increasing molecular weight, reaching at the limit of infinite Mw the value 4.53 K. No isotope effect on the critical concentration has been detected. Using different methods, Θ-temperatures for both PS-CH and PS-CH-d systems were determined. All approaches generate consistent results and a remarkable isotope effect on the Θ-temperature is observed. It is shown that isotopic shifts on UCST and Θ-temperature translate as a red shift of the C-H stretching frequency of cyclohexane on its transfer from a pure liquid to polymer solution at infinite dilution. The effect of pressure on the Θ-temperature has also been analyzed and it appears to mimic that of the pressure dependence of the critical temperatures of polystyrene-cyclohexane solution. It is also shown that the frequency shift of C-H vibrations changes with pressure in a similar fashion.