Light scattering and viscometric measurements were performed for the system isooctanel poly(decy1 methacrylate) of weight-average molecular weight M , = 250000 within the concentration regime of pair interaction between the macromolecules in the temperature range from 25 "C to 100°C. In case of the intrinsic viscosities [q], the pressure was varied up to 3500 bar. Under isobaric conditions, the osmotic second virial coefficients A , , the z-average radii of gyration rL and [q] pass through a maximum, the Huggins coefficient k, through a minimum, with variation of temperature. These extrema indicate a change in the heat of dilution from endothermal at low, to exothermal at high temperature T. The calculation of the temperature dependence of the intrinsic viscosity [q] ( T ) for isochoric conditions does not yield a maximum, provided the pressure at the lowest temperature is chosen close to atmospheric pressure; it can, therefore, be concluded that the change in the sign of the enthalpy is in this case only due to the free volume of the system which increases markedly with increasing T. For pressures larger than 1700 bar, maxima are always observed, no matter whether the temperature is changed isobarically or isochorically. All parameters under investigation are closely interdependent: A one-to-one correspondence exists between [q] and A , at 1 bar, and k , and [q] interrelate linearly for constant T .