In excellent agreement with the available experimental data, results show that mass barotropy is present over ranges essentially involving Type III and IV systems. In addition, since the phenomenon is shown to appear also in Type II systems, it is concluded that mathematical double point (MDP) transitions do not necessarily control barotropic behavior. Finally, the occurrence of double critical end points satisfying the conditions of mass barotropy unequivocally explains the shift of barotropic points along the branch(es) of the liquid-liquid-gas line, as the alkane's chain length increases. (C) 2011 Elsevier B.V. All rights reserved.