Viscosities for the binary mixtures of isomeric butanols or isomeric chlorobutanes with 2-methyltetrahydrofuran were determined at the temperatures 298.15 K and 313.15 K; the viscosities of the mixture of 2-methyl-2-chloropropane with 2-methyltetrahydrofuran were measured only at 298.15 K. The viscosity data were correlated by the McAllister equation. Viscosity deviations for the mixtures containing isomeric butanols are negative over the whole composition range. On the other hand, for the mixtures involving isomeric chlorobutanes, viscosity deviations are slightly negative, except for that for 1-chlorobutane; in this case viscosity deviations present an inversion of sign.