The spatial distribution of chemical species can be a critical determinant of the performance of chemical reactors. One such reactor is the combustion chamber of the Internal Combustion engine. This paper presents a design for the measurement of hydrocarbon concentration distribution within a running engine using near infra-red absorption tomography. The fundamentals of the technique, and design parameters for the equipment are discussed. By utilising micro-optic components, a minimally invasive system is feasible and by utilising advanced laser/photodetector combinations, good temporal performance is anticipated. (C)2000 Elsevier Science S.A. All rights reserved.