  The failure of S and O abundances in most planetary nebulae to display the
same strong direct correlation that is observed in extragalactic H II regions
represents one of the most perplexing problems in the area of PN abundances
today. Galactic chemical evolution models as well as large amounts of
observational evidence from H II region studies support the contention that
cosmic abundances of alpha elements such as O, Ne, S, Cl, and Ar increase
together in lockstep. Yet abundance results from the Henry, Kwitter, & Balick
(2004) database show a strong tendency for most PNe to have S abundances that
are significantly less than expected from the observed level of O. One
reasonable hypothesis for the sulfur anomaly is the past failure to properly
measure the abundances of unseen ionization stages above S^+2. Future
observations with Spitzer will allow us to test this hypothesis.
