The dynamic behavior of counter flow heat exchangers is studied using the harmonic response. To solve the set of partial differential equations the complex notation is used so that these equations become ordinary differential equations. In an appendix it is shown that assuming a uniform temperature in a cross section of the separating wall is correct for the example used as comparison. Experimental results dealing with a double pipe heat exchanger are compared with results obtained by assuming the correlations to be used for the convection heat transfer coefficients. The comparison is satisfactory when the excitation is applied to the inner tube fluid. It is less accurate when the excitation is applied to fluid flowing in the annulus. Finally it is shown that the residence time of the fluids is linked to a peak in the response curves.