Low-level monitoring of atmospheric radioactivity requires sampling with high collection efficiency for the aerosols and gases of interest. The radionuclide station network of the International Monitoring System designed to detect very low-levels of airborne radionuclides will consist of 80 particulate and 40 noble gas stations. This paper reports on studies performed to select suitable filter material and to characterise the sampling efficiency of air inlet configurations of high-volume particulate samplers. It also considers the collection characteristics of automatic xenon sampling systems.