This paper presents the monitoring of acid deposition in Flanders (Belgium) and the change of the measurement objectives, recently towards measurement of ammonia. The monitoring network on acidification has provided results for 10 years. Analysis of acidifying pollutants between 2005 and 2010 shows decreasing concentrations throughout the whole period. NO2 has no significant trend, although the average total deposition in 2010 (2027 Eq/ha.y) is still higher than the objectives of the Flemish environmental policy plan for 2015 (1800 Eq/ha.y). Given the contribution of ammonia, the Flemish Environmental Agency has the aim to develop a policy-oriented monitoring network on ammonia. Studies on this subject have already been extensively conducted.