The present invention relates to heat transfer systems and methods involving zinc-containing surfaces in contact with a working fluid comprising refrigerant, lubricant and a protective agent comprising a compound according to Formula I:

##STR00001##

where R and R.sup.1 are each independently selected from the group consisting of hydrogen, alkyl, hydroxyalkyl and alkylthio, provided that at least one of R and R.sup.1 is alkylthio.