The slow release of reactive gases from the walls and electrodes of practical plasma enhanced chemical vapor deposition (PECVD) systems has been studied. These desorption effects become important when thin films with different compositions are sequentially deposited in single‐chamber deposition systems. The pressure rate of rise of a PECVD reactor was measured after exposure of the system to commonly used precursor gases. The nature of the gas, its pressure, the system temperature, and the time duration of the exposure are experimental variables. Contamination effects by dopants at the parts per million level were investigated by their effect on the electrical properties of subsequently deposited films. It is found that ammonia and diborane, both of which are extensively used for device fabrication, are especially difficult to remove from the system. Operational procedures such as long pumpout times, baking, and short exposure times to the gases, preferably at low pressures, can often reduce the extent of the carryover to an acceptable level in a single‐chamber system. However, the use of multichamber systems is likely to be the most effective solution to crosscontamination problems in PECVD processes.