A simple acoustic method has been used to investigate the two ternary systems CO2 + CH2F2 + CF3CH2F and CO + C2H4 + CH3CHCH2. Vapor-liquid equilibria data were measured for four of the six binary subsystems as well as the two ternary systems. The binary systems containing either CO2 or a refrigerant (CH2F2, CF3CH2F) have been investigated over the whole mole fraction range. For the subsystems containing CO, only the range from 0 to 40 mol % CO could be investigated. The ternary systems were measured by the quasibinary method (i.e. the ratio between two components being held constant). For the system CO2 + CH2F2 + CF3CH2F, three different quasibinary mixtures, with three different compositions each, were measured. For the ternary system CO + C2H4 + CH3CHCH2, one quasibinary mixture with three different compositions was investigated. Additionally, two specific mixtures were investigated in the ternary system CO + C2H4 + CH3CHCH2, because of the importance of this system in the synthesis of aliphatic polyketone polymers. For each mixture the critical point was determined. Different ways of presenting binary and ternary critical data and the topology of the resulting ternary critical surfaces are discussed with respect to thermodynamics. Some limitations of the acoustic method for probing phase behavior are explained. The critical data have been modeled using the Peng-Robinson equation of state.