The average radius of gyration R(g) for the isotopic blend deuteropolystyrene/polystyrene (d-PS/PS) and for the blends deuteriopolystyrene/poly(vinyl methyl ether) (d-PS/PVME) and deuteropolystyrene/poly(phenylmethylsiloxane) (d-PS/PPMS) has been measured by SANS experiments as a function of temperature (T) and pressure (p) up to 1.2 kbar. Furthermore, the specific volume as a function of p and T has been measured. The resulting compressibilities beta(v) = -(1/V)(partial derivative V/partial derivative p) and the compressibility of the radius of gyration, beta R(g) = -(3/R(g))(partial derivative R(g)/partial derivative p), were compared. For d-PS/PS the following were found : (i) For T > T-g (T-g = glass temperature) R(g) changes with pressure less than the macroscopic sample dimensions (beta R(g) < beta(v))) whereas beta R(g) decreases with increasing temperature. (ii) At T-g geometrical affinity is approached (beta R(g) approximate to beta(v)) (iii) Below T-g there is still an increase of the ratio between beta R(g) and beta(v). This implies that in the glassy regime the polymer chain seems still to be mobile on a "local scale. Thus, in general, it is concluded that the "matrix effect" in the chain is relatively small both above and below T-g. The value of the temperature coefficient kappa = partial derivative ln(R(g)(2))/partial derivative T < 0 is consistent with detailed calculations of Yoon et al. For the other blends, where the temperatures are far above T-g, the compressibilities beta(v) and beta R(g) differ strongly and the change of their values with temperature is small.