Abstract: A new liquid-diffusion cell with metallic diaphragm has been applied to investigate the diffusion coefficients of five binary systems containing longchain molecules at various concentrations. The parameter values of the empirical poly-nominal correlating diffusion coefficient with concentration have been obtained by data fitting. A new equation correlating diffusion coefficient with concentration has been derived (22) This equation and other six equations used for comparison have been applied for prediction to 23 systems(207 points)with grand % deviations shown below: * With one empirical parameter It shows that, among the equations without empirical parameter, Eq. (22) of this paper has the lowest value.