The WLF relation connecting viscosity to temperature is used as the basis for a formal description of the compositional variation of blend viscosities. An algebraic manipulation and the use of the cognate pure-constituent Williams-Landel-Ferry (WLF) factors provide a means of introducing composition and pure-constituent viscosities explicitly. Further progress follows through the use of a predictive theory of the compositional variation of glass-transition temperatures for random, single-phase blends. Particular versions of a general relation so obtained are explicated. The theory gives as the simplest case the benchmark linear rule of mixtures, from which it is clear that both positive and negative deviations can arise.