Dew points for ternary methane + ethane + butane and quaternary methane + ethane + butane + water mixtures were determined experimentally between 4.77 X 10(5) and 99.45 X 10(5) Pa and at temperatures from 250.92 to 288.54 K. The experimental dew point curves of the mixtures with water were reproduced quite accurately with an excess function-equation of state method, independent of the temperature-and pressure ranges.