The mutual non-stationary isochoric and isothermal concentration diffusion of the two imperfect neat gases Neopentane(1) and Argon(2) at 59°C is continuously observed in a Loschmidt diffusion apparatus by means of local interferometric gas mixture analysis. Only two diffusion experiments have been performed to get complete information of the isothermal concentration dependence of the binary diffusion coefficient D₁₂(X₁) in the total mole fraction range 0<x₁ <1 of Neopentane. Therefore an extended diffusion equation has to be solved, which considers this concentration dependence and the gas pressure changes on account of gas mixing and gas sorption effects as small perturbations of Fick`s second law. The experimental conditions and the theoretical solution of this diffusion problem are briefly described. The effects of the perturbations as well as of the uncertainties of measuring time t and measuring position Z_A on D₁₂(x₁) are discussed. On account of the exemplary results of this work the used diffusion method may be recommended as a precise and efficient method for measuring D₁₂, absolutely without any calibration of the apparatus and with an uncertainty of better than 1%.