Using an upward thermal diffusion cloud chamber specially designed for operation at elevated pressures and temperatures, we have observed a strong dependence of the homogeneous nucleation of 1-propanol vapor on total pressure. In our investigation, this dependence is manifested in increasing values of the measured S-crit (critical supersaturation) of 1-propanol with increasing total pressure. The increase in total pressure is accomplished by increasing the amount of a carrier (or background) gas in the diffusion cloud chamber. The observed increase in measured S-crit implies that the homogeneous nucleation rate of 1-propanol decreases dramatically as the total pressure increases. We have observed a significant dependence on the nature of the carrier gas. The effect of increasing total pressure on the nucleation of 1-propanol is greater using helium as the carrier gas than when hydrogen is used. We also observe that the dependence of S-crit on total pressure (as manifested by the slope of the S-crit vs P-tot data at constant temperature) is temperature dependent. The measured slope of the S-crit vs P-tot dependence increases as the temperature decreases. In addition, we present S-crit vs temperature data for the 1-propanol/helium system that appear to span the widest range of temperatures to date.