We have investigated the relationship between System III modulation of several Jovian phenomena and their modulation at the 3% longer System IV period. Because these same phenomena also show modulation with a 14.1-day period, there are two possible but physically dissimilar causes of the System IV modulation. The first possibility is that System IV modulation is a separate phenomenon, independent of System III. Alternatively, if System III were amplitude modulated with a 14.1-day period, then the 10.2-hour System IV period would be a sideband of System III, and there would be a second sideband with a period of 9.64 hours. Neither the 9.64-hour nor the 14.1-day periods appear in periodogram analyses of EUV data from Voyager 2 and narrow-band kilometric data from Voyager 1 and 2. Furthermore, we show that System IV modulation cannot be caused by beats or harmonics of periods of System III, Earth's rotation, and Io's revolution. We therefore conclude that System IV is independent of System III. ¿1991 American Geophysical Union