The in-pile performance of MOX fuel is investigated in experimental programmes conducted in the Halden reactor, Norway. The irradiation is performed in rigs allowing steady state, power ramping and cyclic operation. In-pile data are obtained from instrumentation such as fuel centreline thermocouples, pressure transducers, fuel and cladding elongation detectors and gauges for measuring the diametral deformation. Various phenomena can be assessed in this way, e.g. thermal performance, swelling and densification, PCMI and fission gas release. Based on experience with UO2 fuel testing, the Halden Project has defined an extensive experimental programme related to MOX fuels which is being executed in the present programme period. The paper describes the objectives of various experiments and provides examples of temperature, pressure and cladding elongation measurements.