This paper presents a methodology for a rapid determination of important natural gas combustion properties (lower heating value, Wobbe index and the stoichiometric air-fuel ratio) using easily detectable physical properties. It is possible to determine natural gas composition by measuring two physical properties and using specific ternary diagrams (CH4-C2H6-C3H8 and CH4-C2H6-N-2). The first part of the work deals with the selection of two physical properties from a group comprising thermal conductivity, refraction index, and speed of sound. Then, in the second part, a sensor using the best couple of physical property is used to determine the ternary pseudo-constituents of the gas mixture. The model and the sensor are applied to specific situations such as the online determination of LHV. The error on the combustion properties of natural gas is less than 1% over the gases considered in the present study and over about 20 typical gases supplied over Europe. The effect of small errors in the measurement of physical properties has also been highlighted. (C) 2003 Elsevier Science Ltd. All rights reserved.