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The objective of this paper is to introduce a new method for full-field measurement of local average gas-phase fuel concentration in a transient axisymmetrical gas-jet or evaporating spray. Since the combustion process in a diesel engine is a diffusion flame, the local fuel concentration as a function of time is one of the factors that governs engine efficiency and emissions. The method is utilizing the classic Schlieren technique. A CCD camera and frame-grabber combination is used to record the data. Based on these data and the assumption that the flow-field is axisymmetrical, the local index of light refraction is calculated, and from this an estimate of the local gas-phase fuel concentration is made. Since the flow-field is turbulent, data from a large number of separate injections are used. Therefore the results should represent the development of the average flow-field. Since it is a full-field method, the results can be checked for overall conservation of mass.