Ammonia interference in acidimetric determination of sulphur dioxide can be eliminated by separate determination of ammonia in the same sample and by correction of results. However, with this addition the procedure is no longer simple.To avoid two separate determinations a selective absorption of ammonia from the air stream before it enters the absorbent solution for SO2 has been considered. Several absorbents used to remove or to collect ammonia for subsequent determination were tested in the laboratory. Whatman No. 1 (or 41) filter papers impregnated with 3% oxalic acid solution in ethanol proved to be the most efficient for ammonia retention without an effect on SO2. In a comparative testing over 52 days at a permanent sampling station no statistically significant difference was observed between two trains with impregnated filters and a third train where correction for ammonia, separately determined in the same sample, was performed.Regression analysis, however, showed that the results obtained in the third train are more dispersed due to accumulated errors in two analyses at low concentration levels further aggravated by conversion of ammonia to equivalent SO2 concentrations. The removal of ammonia with impregnated filters gives better precision with less inconvenience and is therefore preferable.