During the second year of this project, we developed improved sampling and analysis methods for PNA in diesel exhaust. To do this, the authors worked mainly with one engine at a few fixed speeds and loads and usually confined our emission measurements to two PNA species, namely, BaP and BaA. They used the 14C isotopes of BaP and BaA as tracers to compensate for losses in sampling and analysis. We modified the sampling system that we had developed in the first year by diluting the exhaust sample with chilled air (2 to 4 times by volume). They switched from benzene-methanol to benzene as the filter extractant because pure benzene extracts fewer polar compounds. Isotopic dilution with 14C PNA's proved to be very valuable, even essential, in compensating for the PNA sampling and analysis losses. While the use of 14C PNA tracer compensated for PNA losses in sample collection and analysis and improved reproducibility of the PNA measurements, it did not reduce the PNA loss. The authors do not yet know whether these losses occur principally during sample collection or in analytical procedure.