In melts, ring polymers assume more compact conformations compared to linear chains with the same degree of polymerization Nr, i.e. R r ∼ Nrv with v ≈ 0.4, where R r is the radius of gyration. Upon gradually substituting some of the ring polymers with linear chains, the ring molecules swell. In the limit of infinite dilution, their size scales as Rr ∼ Nr 0.5. We present a scaling argument based on the blob model to capture this transition. Ring-linear blends are modeled as a semidilute solution of ring polymers in a Θ-solvent consisting of linear chains. The model predicts that the size of the ring polymer remains unchanged up to the overlap concentration cr*. Beyond cr* the size of the ring shrinks according to Rr ∼ crβ, where β = 2v - 1 = -1/5 for v = 0.4. The overlap concentration depends on the degree of polymerization of the ring according to cr* ∼ 1/√Nr. These predictions were tested by performing Monte Carlo simulations of ring-linear blends using the bond-fluctuation model. The results of the simulation for Nr = 150 and 300 blended at different concentrations with linear chains of the same degree of polymerization validate the scaling model. © 2007 American Chemical Society.