The phase behaviour of binary mixtures of cyclic poly(dimethylsiloxane) (PDMS) and linear poly(methyl-phenylsiloxane) (PMPS) was investigated using a static wide-angle light-scattering technique. Evidence for the effect of component molecular architecture/topology (cyclic or linear) on the phase separation of blends was sought through cloud point measurements and by examining the composition dependence of the resulting phase diagram. The cyclic-linear blend exhibited an upper critical solution temperature (UCST) and showed a dependence on the molecular architecture/topology in that the critical temperature T(c) for the cyclic-linear blend was lower than expected when compared with PDMS-PMPS blends in which both components had a linear polymeric structure. The interaction energy density parameter LAMBDA12 for the polymers was evaluated via the Flory-Huggins treatment for the thermodynamics of mixing of two polymers. The blending of cyclic PDMS led to a lower LAMBDA12 when compard with the corresponding linear-linear binary blend, again because of the chain architecture/topology effect. For this UCST system, it thus appears that PDMS cyclics are more soluble/compatible than their linear analogues of the same degree of polymerization. These findings are consistent with the current, but very limited, experimental data in the literature for large ring systems.