Reports published by Gulf R&D Co. and Battelle Columbus Laboratories under contract to the Coordinating Research Council's APRAC project group CAPE-24 were reviewed. Both studies failed to verify the accuracy of polynuclear aromatic hydrocarbon (PNA) emission measurements from heavy-duty diesel engines. Thermal decomposition and chemical reactions of the PNA occur in raw exhaust at temperatures above 500°F. Therefore, pipes which transfer exhaust to dilution tunnels can significantly reduce the apparent emission values. Dilution tunnel conditions have comparatively little effect on PNA measurements. However, vapor traps are required behind particle filters to assure complete collection of 4-ring PNA compounds. Guidelines are presented for controlling and testing sampling systems for accurate PNA emission measurements.