Exhaust particulate and gas composition samples were obtained at various distances along an externally air cooled exhaust from a Perkins 4-236 single cylinder engine. The change in the particulate composition was determined as a function of the exhaust distance and local temperature. Exhaust temperatures were in the range 200 - 260C at entry to the tunnel at all engine conditions. A constant filter paper and sample temperature of 50C was used for both exhaust and dilution tunnel samples and the filter paper was mounted in an oven for this purpose and the particulate sample was tranported through heated lines to this oven. Associated with these particulate measurements were gas analysis measurements. UHC were measured at 180, 50 and 2C in the exhaust and the differences were taken as an indication of the condensable hydrocarbons over that temperature difference. The results showed that at low powers there was a continual reduction in gas phase UHC and increase in particulate SOF as the exhaust gases were cooled along the exhaust pipe. At high power and maximum exhaust temperatures there was an initial consumtion of UHC and PAH followed by an increase in PAH and UHC which was concluded to be due to volatile release from exhaust pipe deposits. Analysis of the deposits showed the presence of PAH and C16-C27 n-alkanes. There was an increase in the total particulate SOF and of some PAH compounds between the exhaust and tunnel, but the majority of compounds had the same mass emissions in the cooled exhaust and tunnel. There was no evidence of any change in the particulate composition or mass flow along the tunnel.