We present results of a small-angle neutron scattering (SANS) study of the structure and thermodynamic properties of symmetric blends of deuterated poly(dimethylsiloxane) (d-PDMS) and poly(ethylmethylsiloxane) (PEMS) as a function of temperature (T) (40less than or equal toTless than or equal to300degreesC) and the molecular weight (M-w) (4700less than or equal toM(w)less than or equal to23,200). The radius of gyration (R-g) of d-PDMS was measured using the high-concentration labeling method and revealed unperturbed chain dimensions at all temperatures regardless of the polymer M-w. The random phase approximation (RPA) fits the data for low M-w blends, however it fails to describe the SANS data for M-w > 10,000 g/mol. This observation is explained by the fact that for high M-w blends the correlation length of the concentration fluctuations xi is always large (xi > R-g), implying that these blends remain microscopically inhomogeneous at all temperatures studied in this work. At the same time, the low Mw blends are randomly mixed (xi < R-g) at all T and can reach the 'ideal mixing' or Theta condition (chi = 0). (C) 2004 Elsevier Ltd. All rights reserved.