The enthalpies of reaction of neat PBu3 with solid sulfur (-27.1 +/- 0.5 kcal/mol), selenium (-20.0 +/- 0.6 kcal/ mel), and tellurium (-4.9 +/- 0.6 kcal/mol) have been measured by solution calorimetry. The enthalpies of reaction of a series of phosphines with sulfur in toluene solution have been measured as follows (values in kcal/mol) : PCy3 = -30.9 +/- 1.9, PBu3 = -28.9 +/- 0.3, PMe3 = -27.1 +/- 0.4, PMe2Ph = -26.0 +/- 0.5, PMePh2 = -23.8 +/- 0.3, PPh3 = -21.5 +/- 0.3. These values correlate with literature data for enthalpies of protonation and indicate that P to S sigma donation is probably the dominant factor in determining the R3P=S bond strength, estimates for which range from 88 to 98 kcal/mol. The enthalpies of S atom transfer to PPh3 by S=AsPPh3 and S=SbPh3 in toluene solution are -17.7 +/- 1.2 and - 21.5 +/- 1.0 kcal/mol, respectively. The enthalpy of removal of the central S atom from BzSSSBz by PCy3, yielding BzSSBz and S=PCy3, is - 29.0 +/- 1.8 kcal/mol. These data are used to establish a range of enthalpies of S atom transfer in these compounds which spans 31 kcal/mol from S=SbPPh3 to S=PCy3.