Abstract When magmas of different temperature and composition are intimately mingled together, transfer of heat results in substantial changes in the rheological properties of the magmas. Since thermal diffusion rates are orders of magnitude faster than chemical diffusion rates, mixing magmas will come nearly to the same temperature before complete homogenization of the magmas can occur by diffusion and shearing. The ability of magmas to mix thus depends on their physical properties after thermal equilibration. Calculations are presented on how the viscosity and crystal content of mafic and silicic magmas vary as a function of their initial temperatures and the proportion of mafic magma in the mixture. Three physical situations can be identified: (a) where the mafic magma remains less viscous than the silicic magma; (b) where the mafic magma becomes more viscous than the silicic magma due to crystallization; and (c) where the mafic magma is effectively solid due to its high crystal content. In the last situation it is proposed that complete mixing cannot take place, but the mafic magma is dispersed as solid xenoliths or inclusions within the silicic magma. Xenolith or inclusion formation occurs when there is a large temperature difference between the magmas or a large proportion of silicic magma. Complete hybridization can only occur when the magmas both behave as liquids at the same temperature. A diagram is constructed that shows the fields where the mafic magma becomes a solid or remains a fluid on a plot of the proportion of mafic magma against the composition of the mafic magma. Where there is a large proportion of silicic magma, complete hybridization can only occur with evolved mafic magmas (andesitic magmas). An example of this compositional selectivity is described from St. Kilda, Scotland where silicic magmas have only hybridized with highly evolved theoleiitic andesite magmas, although the silicic magma is intimately intermingled with more mafic magmas in net-veined complexes. When basaltic magma is intermingled with silicic magma, high proportions (typically 50% or greater) of basalt are necessary to enable mixing to occur. Hybrid magmas involving small proportions of basalr magma with large proportions of silicic magma are notably absent from St. Kilda hybrid rocks and other areas. Many mafic xenoliths may represent less evolved basaltic magma which solidified when commingled with a much larger volume of silicic magma.