Concurrent gas and fog water measurements of formic, acetic and pyruvic acids and formaldehyde carried out during the Po Valley Fog Experiment 1989 are presented. The reaction between HCHO and S(IV) in fog water solution to form HMSA was studied. The effect of HMSA formation was found to increase HCHO solubility in fog water up to 100 times. 95% on average of HCHO is present as HMSA in fog water samples with pH > 4.5. At lower pH values, HMSA formation is limited by the availability of S(IV) in solution. A common feature of HCOOH, CH3 COOH and HCHO gas/liquid distribution is represented by the large departures from theoretical predictions when fog water pH is in the region where a large fraction of these compounds is partitioned into the liquid phase. A limitation in mass transport across the air/droplet interface is a likely explanation for this behaviour.