Preliminary results from a study characterizing S(IV) compounds in wintertime precipitation samples indicate that bisulfite ion is not the primary form of S(IV), as previously believed. By employing a differencing technique that permits estimation of both SO2· aq and non‐SO2· aq compound concentrations, it was found that, on an average, more than 60% of the total S(IV) is present in a form other than dissolved SO2. Formaldehyde analyses on selected samples suggest that the most likely form of the S(IV) is hydroxymethanesulfonate, although other aldehyde‐S(IV) adducts may also be present. The non‐SO2‐compounds represented a significant portion of the total sulfur concentrations present in the samples analyzed, with contributions ranging from 1.2 to 27%. Because of the stability and oxidation resistance of these S(IV) compounds, sulfur deposition estimates that are based solely on sulfate measurements are undoubtedly low, especially for wintertime events. The study underscores the importance of S(IV) compounds in atmospheric scavenging