The chemical composition of the surface boundary layer over the Eurasian continent is still an area of high uncertainty. In the framework of the Trans-Siberian Observations Into the Chemistry of the Atmosphere (TROICA) project, measurements of O3, NO, NO2, CO, CO2, CH4, 222Rn, J(NO2), and black carbon aerosol were carried out on the Trans-Siberian railroad during June--July 1999. Boundary layer data over more than 16,000 km, from Kirov (~58¿N, 49¿E; 972 km east of Moscow) to Khabarovsk (~48¿N, 135¿E) and back to Moscow, were obtained without significant contamination, emphasizing the potential of using the Trans-Siberian railroad system for atmospheric measurements. The 222Rn and CO2 concentrations were determined for the first time using our laboratory wagon. The diurnal variations of these gases and of CH4 due to micrometeorological conditions, as well as their dependence on various soil sources and vegetation types, were used to estimate ecosystem fluxes of CO2 and CH4. The highest soil flux of CH4 was 70 ¿ 35 ¿ mol m-2 h-1 for the wet habitats of the West Siberian lowlands, and the lowest CH4 flux was 3.2 ¿ 1.6 ¿ mol m-2 h-1 for drier habitats of eastern Siberia. Although the wet tundra emissions found between 67¿ and 77¿N are higher than in comparable environments at much lower latitudes <Christensen et al., 1995>, boreal wetlands in Siberia at 50¿--60¿N represent a very important player in the global methane budget. The CO2 density fluxes exhibited the opposite to CH4 fluxes tendency. Ozone mixing ratios varied from a few nmol/mol during nighttime inversions to more than 60 nmol/mol during the day. These values were generally higher than during the 1996 summer campaign (TROICA 2). CH4 and CO levels followed the pattern observed during TROICA 2; elevated levels of CH4 with a mean mixing ratio of 1.97 ¿ 0.009 ¿mol/mol were found over the West Siberian lowlands, decreasing to 1.88 ¿ 0.13 ¿mol/mol toward East Siberia. Conversely, while background CO levels of the West Siberian wetlands were generally below 140 nmol/mol, high CO concentrations, once even exceeding 2 ¿mol/mol, were registered east of Chita (~52¿N, 113¿E), as a consequence of forest and other vegetation fires, which significantly affect the chemical composition of the air over parts of Russia.