A conveniently sized high-pressure scattering cell used for 90 degrees DLS (dynamic light scattering) measurements on polymer solutions is described (0 < P/MPa < 200; 283 < T/K < 373). DLS parameters are reported for semidilute solutions of polystyrene (PS) of various molecular weight in Theta-solvents (cyclohexane (CH) and methylcyclohexane (MCH)), above and below T-Theta, and at high enough concentration to ensure that precipitation, when it occurs, follows spinodal decomposition/percolation. In the homogeneous region, close to and above T-Theta, the correlograms, initially monomodal and diffusive, split at high pressure. The intensity and the correlation radius of the diffusive mode diverge as critical demixing is approached during pressure or temperature quenches. That process is described in the (T,P)(psi cr) plane using a multidimensional reduced scaling formalism developed for the purpose. The analysis of the scattering and the phase equilibrium data for PS/CH is complicated by the pressure dependence of the CH freezing curve, but that for PS/MCH nicely exemplifies behavior across a good part of the (T,P)(psi cr), reentrant miscibility island showing multiple hypercritical points. It has been discussed in that context.