The complicated effects of pressure on packed column distillation efficiency are not well understood. We have analyzed a variety of total reflux distillation data, taken from several larger-scale packed columns, and have quantified the effect of pressure on HETP. Our results are supported by theoretical analysis. The effects of vapor and liquid side mass transfer resistance, residence time, liquid rate on mass transfer area, and temperature on key physical properties are addressed. The effects of vapor phase backmixing are also discussed. The final results is a set of guidelines useful to the process design engineer.