In this article a probabilistic model able to describe gel formation in negative tone chemically amplified resists (CARs) is developed. This model is a continuation and development of previous work on polymer cross-linking and on gelation of negative tone nonchemically amplified resists. The relation between gel fraction and exposure dose in negative tone CARs is obtained. A new parameter called inhibition activity with a vital role in the quantification of the cage effect (i.e., acid species trapping) is analyzed. The description of cage effect evolution with exposure dose, postexposure temperature, and time in CARs is accomplished through this parameter. The theory is tested against two negative tone CARs. The first one is EPR, an experimental CAR with known chemistry and the other is SAL-601, a commercial CAR from Shipley.