A diagnostic technique for determining fluid motion and mixing in fired heterogeneous internal combustion engine cylinders was developed and tested on a single-cylinder, open-chamber diesel. A rapid-acting valve was used to inject a small quantity of radioactive tracer, argon-41, into the combustion chamber at a given location and crankangle, once per cycle. A fast-acting sampling probe was used to map the movement and dispersion of the tracer. Data was taken for both fired and motored conditions at low engine speeds. The data showed the technique can be used to quantify gross fluid motions such as swirl and identify manifestations of turbulent mixing.