We report on studies of ion-induced nucleation in a corona discharge taking place in an atmosphere containing SO 2 , NH 3 , and H 2 O at standard temperature and pressure. Positive ions such as H 3 O + (H 2 O) n , NH 4 + (H 2 O) n , and H + (H 2 SO 4 )(H 2 O) n and negative ions such as HSO 5 − (H 2 O) n , SO 4 − (H 2 O) n , HSO 4 − (H 2 O) n and NO 3 − (H 2 O) n have been recorded. Large values of n (>100) were observed and the experiment indicates the existence of even larger water clusters. In contrast, only clusters with a maximum of 2 sulfuric acid molecules were observed. Fragmentation studies also revealed that the negative ion HSO 5 − , which has been observed in many studies, in our experiments is contaminated by O 2 − (HNO 3 )(H 2 O) ions, and this may also have been the case in other experiments. Finally an ion with m / z =232 (where m is the cluster mass in amu and z is the charge state), capable of attaching H 2 O-molecules was observed and studied by fragmentation.