In this study the transient response of counterflow and parallel-flow heat exchangers having finite wall capacitance is investigated. Three configurations are considered: the flat-plate exchanger with phase change in either channel and the shell-and-tube exchangers with phase change either in the tube or in the shell. The transient response of these configurations results from sudden change in the inlet temperature of the one-phase fluid or from sudden exposition of the shroud to ambient conditions. The thermal behavior of the exchanger is described by detecting the transient variations in the temperature of the one-phase fluid and in the quality of the two-phase fluid within the exchanger. Closed-form expressions for these parameters are presented. These expressions can be used to evaluate different design parameters required by heat exchanger designers.