AbstractDilute solutions of many simple binary mixtures conform to the viscosity relationship:— ηs=η0(1+kθ) Where η5 η0 are the viscosities of solution and solvent respectively, θ is the concentration and k is a constant. The constant k has an approximately linear relationship to the molecular weights of the solutes but varies for different homologous series. Using the I renkel relation for the viscosity of liquids η0=GT0 U/RT it is suggested that a solute may affect (a) G or (b) U or (c) both. When solute and solvent have approximately the same times of relaxation, k is not dependent on temperature but in other cases it is. Under conditions of iso‐viscosity, i.e., for equal values of U/RT dilute solutions of cellulose nitrate in various solvents and solutions of naphthalene in benzol and toluol yield approximately equal values for 1/θ (η6/η0‐1) for a given solute.