In this work we show that the shape of the liquid-vapor saturation curve in a T-r-s* diagram (T-r = T/T-c, and s* = s/R, with T-c the critical temperature, s the molar entropy and R the gas constant) for a given fluid is mainly governed by the acentric factor, omega, and the critical molar volume, v(c), of the fluid. The study uses as reference the point M where the saturated vapor curve in the T-r-s* diagram changes its concavity, i.e. (d(2)s*(g)/dT(r)(2))(M) = 0. By analyzing the data provided by the National Standards and Technology (NIST) program RefProp 9.1 for 121 fluids, we find that, at this point, T-Mr approximate to 0.81 and the slope xi(M)* = (ds*(g)/dT(r))(M) is well correlated with nu(c), existing a threshold value nu(c.0) approximate to 0.22 m(3) kmol(-1) so that xi(M)* < 0 (wet fluid) for nu(c) < nu(c,0) and xi(M)* > 0 (dry fluid) for nu(c) > nu(c,0). This direct relation between v c and the wet or dry character of a fluid is the main result of the present work. Furthermore, the dimensionless vaporization entropy at the reference point M, Delta v = s(M)*(g) = s(M)*, increases in a nearly linear way with omega. (C) 2018 Elsevier Ltd. All rights reserved.