The densities and the viscosities of four binary non-electrolytes at 25.00°C have been measured. The mutual-diffusivities of two systems and the intra-diffusivities of three systems have been determined with a high degree of precision over the entire composition range.

The development of the new diaphragm cell calibration technique resulted in a higher degree of reproducibility of the cell constants than before.

The various theories of diffusion in liquid mixtures have been subjected to critical testing using the diffusivity and viscosity data obtained in this study.

A new equation to predict the dependence of the mutual-diffusivity on concentration based on Eyring's theories of diffusion and viscosity has been derived and tested using the data obtained in this study and data on twenty other systems reported in the literature. The equation appears to be superior to existing correlations. The proposed equation could predict reasonably well the diffusivities of the polar mixtures for which data were available.

A new feature of this equation, is that it utilizes the quotient of the diffusivity and viscosity rather than the product of the diffusivity and the viscosity.

The new equation in its original form was found to be less successful in the case of systems where both components are straight chain hydrocarbons, but using different mixing rules for the free volumes and molecular weights improved the predictions considerably.

The friction coefficient approach to the diffusion problem based on both irreversible thermodynamics and statistical-mechanics has been tested and discussed in some detail in this study. It has been shown that Lamm's friction coefficients are more physically meaningful than those obtained from the statistical-mechanical theory. Lamm's friction coefficients have been calculated for the systems under investigation and were utilized to investigate qualitatively the spatial distribution of molecules in the liquid mixtures.