The goal of a fault diagnostic system is to detect any incipient abnormal situation in a plant operation and to find out the fault that causes that situation, e.g. a blocked valve or a failed pump. In such a task, time is highly restricted and the system must be capable to help the operator in correcting the abnormal situation as quickly as possible, in order to achieve a stable and secure operation of the plant. There are a few previous works about diagnosis for batch processes. The main problem is that batch processes evolve along the normal operation. Tarifa and Scenna (1995) presented a method destined only to batch bioreactors. In this work, a more general method is presented and it is applied on a batch reactor.