A test rig has been assembled to investigate the thermodynamic and hydraulic performance of a cylindrical graphite block heat exchanger consisting of three graphite blocks and a steel shell. The flow pattern in the heat exchanger was triple cross-flow on the shell-side (service side) with one pass per block and a single pass on the tube-side (process side). Overall heat transfer coefficients and pressure drops have been measured for a range of operating conditions. Shell-side leakage flows were significant, with values up to 19% with water and 38% with a heat transfer oil. Corresponding increases in the overall heat transfer coefficient without leakage flow were 8 and 21%, respectively. The mean difference between the theoretical and experimental overall hear transfer coefficients for the runs with leakage flow does not exceed 6% with water as the shell-side liquid. With oil the error does not exceed 8%, Pressure drop models were developed for both the shell-side and tube-side of the heat exchanger. The tube-side model predicted the pressure drop with a mean error of 5%. Errors within 20% can be expected for the shell-side pressure drop model. The pressure drop on the shell-side depends on a number of different losses and it is therefore difficult to be modelled accurately.