This paper presents a methodology for the determination and measurement of important natural gas combustion properties (Higher heating value, Wobbe index and Stoichiometric Air-fuel ratio). A pseudo-gas formulation is used to determine an equivalent gas composition to the real natural gas tested. The pseudo-composition, made up of the most influent constituents of the natural gas, is determined by solving a system of non-linear equations. The input parameters to the procedure are: the thermal conductivity of the natural gas at 333 K and 383 K, the speed of sound at 303 K and the concentration of CO2. A combustion properties measurement sensor has been developed and tested for many natural gas compositions. The tested natural gases are chosen to represent typical European gases as well as to account for large variations of individual components (heavy hydrocarbons, inert gases). With the developed sensor, combustion properties measured at standard conditions agree with gas chromatographic analysis, to within 0.5%. (C) 2007 Elsevier Ltd. All rights reserved.