This paper reports measurements made for DIPPR Research Project 821 in the 1996 Project Year. Vapor pressures were measured to a pressure limit of 270 kPa (unless decomposition occurred) for all six compounds using a twin ebulliometric apparatus. Additionally, for c-caprolactam, measurements at low pressures (0.043 kPa to 3,0.8 kPa) were performed using an inclined-piston apparatus. Liquid-phase densities along the saturation line were measured for each compound over a range of temperatures (ambient to a maximura of 548 K). A differential scanning calorimeter was used to measure two-phase (liquid + vapor) heat capacities for each compound in the temperature region ambient to the critical temperature or lower decomposition point. A critical temperature and the corresponding critical density were determined experimentally for pyrazine. The results of all the measurements were combined to derive a series of thermophysical properties including critical temperature, critical density, critical pressure, acentric factor, enthalpies of vaporization (within the temperature range (+/-50 K) of the vapor pressures), enthalpies of fusion if solid at ambient temperature, solubility parameter, and heat capacities along the saturation line. Wagner-type vapor-pressure equations Were derived for each compound. In addition, the liquid-phase densities were compared with values derived using a four-term power series in [(1 - T-r)(n/3)]. For epsilon-caprolactam, the results of the present measurements were combined with literature values to derive a "Third Law" estimate of sublimation pressures in the region of ambient temperature. All measured and derived values were compared with those obtained in a search of the literature.