This article presents the design, fabrication, and calibration of thermocouple pairs for local heat flux measurement. The intended application of the thermocouple pairs is on the tubes of phase-change heat exchangers experiencing heat fluxes on the order of 705 W/m(2). Particular advantages of this technique are that it is accurate even for thin-wall tubes, there are no restrictions on the tube inclination, and it is economical. A thermocouple pair consists of a sheathed thermocouple which measures the tube outer surface temperature and a novel plug-type thermocouple which measures the tube inner surface temperature without interrupting the heat transfer phenomena A calibration was performed by imposing a series of known heat fluxes on a condenser tube that had been outfitted with a number of thermocouple pairs and measuring the temperature difference between the thermocouples. The maximum uncertainty associated with the calibration constants was +/-9.5%. This tube was then used as the condenser tube of a double-pipe heat exchanger to obtain local heat flux data that had previously been unattainable around a horizontal tube.