A unique feature in the CO2-H2O-1-butanol system is the formation of four fluid phases in thermodynamic equilibrium. Investigations into this phenomenon now permit a complete description of phase behavior between 296 K and 315 K at pressures ranging from 5.8 MPa to 11.0 MPa, The experimental programme covers the determination of equilibrium compositions in fluid three-and four-phase equilibria as well as measurements of critical lines and critical endpoints. Experiments were selected in order to prove Gibbs' theorems on the formation of four-phase equilibria in ternary systems. Calculations using the Redlich-Kwong-Soave equation of state in combination with mixing rules proposed by Huron and Vidal complete the description of phase behavior in regions hardly or not accessible by experiment. The model, at least qualitatively, is capable of predicting the full extent and variety of the phase behavior for CO2-H2O-1-butanol mixtures from binary two-phase equilibrium data alone. Considerable improvement towards a quantitative description, however, can be achieved when p,T-data along the four-phase line of the system is incorporated in the evaluation of the interaction parameters of the empirical mixing rules.