Phase equilibria in the n‐hexadecane‐water‐hydrogen and n‐hexadecane‐benzene‐water‐hydrogen systems were determined experimentally at temperatures between 200 and 350°C and pressures between 100 and 300 bar. At high water concentrations, three‐phase equilibria were observed. Two‐phase regions could be correlated with a modified Redlich‐Kwong equation of state. The influence of interaction parameters on the calculated miscibility gaps was investigated. On application of mean interaction parameters, it is possible to calculate phase equilibria at different pressures or temperatures with the same set of parameters. In the ternary system, the three‐phase regions could be calculated from the correlated binodal curves of two‐phase regions. In the quaternary system, cross‐sections through the vapour‐liquid miscibility gap could be successfully correlated.