Species yields and combustion efficiency of lab-scale flares, turbulent non-premixed buoyant flames, were measured. A new facility was constructed allowing gas mixtures to be burned on 38.1 to 76.2 mm diameter flares at flow rates up to 410 standard litres per minute. A methodology was developed to quantify species yields and combustion efficiency within calculated uncertainties. Results showed combustion efficiencies greater than 97.8% in all cases, with up to 90% of the non-CO2 carbon emitted as soot. Soot yields were heavily dependent on flare gas chemistry, ranging from an average of 7.34∙10-5 kg-soot/kg-flare-gas for methane tests to 1.20∙10-2 kg-soot/kg-flare-gas for ethylene tests. NOX data suggest that average mass yield per energy content of the flare gas is 3.76∙10-2 kg-NOX/GJ and is independent of exit conditions and fuel chemistry for the range of fuels considered. Results are compared with published soot and NOX emission factors and potential scaling methods are discussed.