The precision of (p, rho, T) measurements on fluids suffers from the effects of impurities, of physical and chemical adsorption and desorption at the walls of the measuring vessel, and of the chemical reaction of the substances at higher temperatures. A study of the low-density volumetric behaviour of steam in a large temperature range performed by our group some years ago showed these difficulties in the evaluation of the experiments, too. New measurements on n-alkane vapours and their analysis with respect to a physically reasonable behaviour of isothermal data prompted new evaluation of the older measurements on steam. Influences of physical adsorption and of chemisorption are particularly taken into account in order to extract reliable values of the second virial coefficient B in the temperature range 381 K to 524 K. A consistency test including recommended values of the density and pressure for the saturated vapour proves the high quality of these data for the second virial coefficient, which are more positive than B values from the literature derived from experimental (p, rho, T) data in the same temperature range.