Information about differences in flame characteristics and consequent modifications required on combustors is essential to combustion engineers for using low-calorific-value gases in place of conventional fuels. This paper presents an experimental study directed to determining the changes in flame characteristics of gas-jet flames when the heating value of the fuel stream is decreased. Both burner-attached and lifted-off flames have been investigated. The reported data include flame length; temperature profiles; flame radiation; and concentration profiles of fuel, pyrolysis species, major products of combustion, and environmental pollutant species (particulates, nitrogen oxides, and carbon monoxide). The practical implications of the results are also discussed.