Laboratory and field studies were conducted to evaluate the collection capacity of the nylon filter (Nylasorb membrane, 47 mm diameter, 1.0 mum pore size, Gelman Laboratory, USA) for HCl and HNO3. In ambient sampling, the field results show that one piece of the nylon filter in the filter pack of an annular denuder system (ADS) or a honeycomb denuder system (HDS) has enough capacity to adsorb HCl and HNO3 gases evaporated from the particles collected on the Teflon filter. The laboratory results show that the individual capacity of the nylon filter to adsorb HCl gas is 8, 160, and 240 mug and that of HNO3 gas is 160, 1400, 1900 mug at 20% +/- 5%, 55% +/- 5%, and 80% +/- 5% relative humidity, respectively. At low humidity, the efficiency of the nylon filter for HCl gas will be decreased substantially when HNO3 co-exists. It is decreased to as little as 35-50% when relative humidity is low, at 20% +/- 5% for the added amount of HNO3 is from 40 to 145 mug. A monolayer adsorption theory was successfully employed to estimate the reduction of adsorbing capacity to adsorb HCl gas when HNO3 gas co-exists. There is no significant effect on the adsorbing efficiency of HNO3 when HCl co-exists; the collection efficiency is greater than 95% at all relative humidities.