A near-critical binary model fluid system consisting of approximately equal amounts of methane and n-decane by weight, is studied by use of interface light-scattering. The main objective has been - in the near-critical region - to obtain a high-accuracy data set for the interfacial tension. The system is studied at constant temperature 37.8 degrees C (100 degrees F) in the pressure range (34.5-36.2) MPa. The upper pressure is only 0.0014 MPa below the critical pressure of the system. In the pressure range studied, the interfacial tension decreased four decades from 1.6 x 10(-1) mN m(-1) to 1.4 x 10(-5) mN m(-1). The lower value is two orders of magnitude lower than measured for any binary hydrocarbon fluid system hitherto. In part of the pressure range also values for the density difference between liquid and gas and the sum of liquid and gas viscosities are determined from the same light scattering data. For interfacial tension and density difference the critical scaling exponents are determined to be respectively 1.304 +/- 0.010 and 0.376 +/- 0.016. (C) 2018 Elsevier B.V. All rights reserved.