The temperature transient response of a single-phase fluid and a wall in a heat exchanger is investigated for when the other constant temperature fluid is subjected to a step change in temperature or when the single-phase fluid is subjected to a step change in mass flow rate. The dynamic behavior of the heat exchanger is approximated by an integral method assuming that the single-phase fluid temperature distribution can be expressed by a combination of the initial and final temperature distributions and a determined time function. The results are validated by comparison against numerical simulations. Excellent agreement is obtained. (C) 2003 Elsevier Science Ltd. All rights reserved.