A tandem-column ensemble consisting of two capillary columns with different stationary-phase chemistries and computer-controlled carrier-gas pressure at the column junction point is used to change the ensemble selectivity on-the-fly during a separation. When the pressure at the column junction point is changed, differential changes occur in the carrier gas velocities in the two columns. This changes the relative contribution that each column makes to the ensemble selectivity. This programmable selectivity control is applied to the problem of high-speed, isothermal GC analysis of volatile organic compound (VOC) mixtures. For the ensemble configuration described here, the pressure control system provides over 200 computer-selected junction-point pressures in 0.1 psi steps. Repeatability typically is ±0.01 psi. This provides for very stable retention patterns. The relatively large dead volume in the controller is managed by the use of a pneumatic vent line. The vent line restriction is set to give pressure set-point transition times of 0.3−3 s. A model is developed to aid in the selection of the pressure program (pressure vs time profile) that will result in a rapid separation of the target compounds. The model results in plots of the position of component bands in the column ensemble vs time. These sample-band trajectory plots show discontinuous changes in slope (migration velocity) at pressure set-point change times and when each component band migrates across the column junction. The model provides quantitative information on elution peak positions. The model is used to predict the effects of changes in the selectivity program on retention patterns.