The objective of this work was to establish whether two detergent-type additives(A and B) influence the drop size and evaporation of two Diesel fuels (1 and 2) under Diesel engine conditions. Two experiments were performed: visualization of liquid and vapor fuel by the exciplex technique in a motored single-cylinder engine and measurement of the Sauter mean diameter, total drop cross sectional area and total drop volume by laser diffraction in a spray chamber. The same Diesel injector and pump system were used in the two experiments. The engine tests were carried out using a high aromatic content fuel (1) particularly suited for the exciplex studies. These studies showed that additive A yielded a lower vapor signal than additive B, which in turn gave a lower vapor signal than untreated fuel. Spray chamber results were obtained for both fuel 1 and 2. Additive A reduced the evaporation of fuel 1 whereas additive B gave a smaller and less consistent affect. For fuel 2, which is more typical of modern Diesel fuels than fuel 1, neither additive had measurable effects on the vaporization of the fuel. These measurements were difficult and were not quantitative. However, it is striking that two different experiments and experimental techniques indicated that additive A reduced the vaporization of fuel 1. However, we were not able to establish whether the reduced vaporization was due to altered atomization or to changes in the vaporization itself. In contrast the same experimental technique showed no changes in vaporization for fuel 2. More detailed investigation is required to understand this unexpected result.