The thermometric titration method presented here permits a simple and direct determination of the total acid number (TAN) in petroleum products. It is an invaluable alternative to current manual and potentiometric methods. Thermometric titration uses a maintenance-free temperature sensor that does not require rehydration and is free of fouling and matrix effects. The procedure requires minimal sample preparation. Results agree closely with those from the potentiometric titrimetric procedure according to ASTM D664, but the thermometric titration method is far superior in terms of reproducibility and speed of analysis, with determinations being complete in approximately one minute.