A dynamic, nonisothermal, nonequilibrium model is developed to predict detailed transient behavior during startup and transition to standby for concentrated random-packed absorbers. The model accounts for solvent evaporation and transient hydraulic behavior and is used to simulate the aqueous absorption of gaseous HCl. During startup, the simulations show a large decrease in the gas flow rate followed by a recovery due to high rates of solvent evaporation. The model predicts a spike in the solute composition in the gas during the startup transient. At low mass-transfer rates, this spike can cause unacceptable solute levels in the vent. During the transition to standby operation (where feed gas is replaced with carrier gas), there is an appreciable increase in the gas flow rate and pressure in the column. The peak transient gas flow rates are about 3 times greater than the feed rate, and the peak pressure drops are about 20 times greater than those at flooding. These spikes may be too brief to flood the column, but mechanical damage is possible. The transient also shows solute being desorbed from the bottom part of the column and re-absorbed in the upper part of the column, which causes a burp of solute in the vent gas. In less concentrated columns, the burp can result in unacceptable solute levels in the vent. These problems can be addressed by initially reducing the carrier gas flow rate. The model also applies to the use of scrubbers for the control of toxic vent emissions.