Chemical properties of an antimicrobial substance isolated from Houttuynia cordata, a popular medicinal plant in eastern and southeastern Asia, were investigated and it was found to contain decanoyl acetaldehyde. Synthesis of the substance was achieved from methyl nonyl ketone and ethyl formate by means of the Claisen condensation. In the course of this work, it was also established that sodium bisulfite adduct of enolizable aldehyde can be prepared by leading sulfur dioxide into the sodium salt solution.