The phase boundary of the binary liquid system CS2+CH3NO2 is studied over nearly six decades in reduced temperature 3×10−6<ε=(TC−T)/TC<2×10−1 and over the composition range 8-98 mole % of CS2. The critical parameters areTC=335.132K andxC=57.34 mole % of CS2. A single critical exponentβ=0.315±0.004 fits the observations over the entire range with no indication ofβ increasing to the classical value of 1/2 far away fromTC. The diameter of the coexistence curve shows a rectilinear behaviour only far away fromTC. NearTC, the deviation ΔX from the rectilinear law seems to fit a curve of the form ΔX=fε7/8 exp (−gεh), the derivative of which has a singularity like that of specific heat. An ambiguity in the analysis of the data in terms of mole fractions and volume fractions is pointed out. It is also suggested that the curvature of the diameter may be much weaker in a liquid-gas system and hence might have escaped detection.