A clinical boron neutron capture therapy (BNCT) facility, employing a high intensity epithermal neutron beam, has recently become available at the High Flux Reactor (HFR) in Petten. Another epithermal beam has been constructed at the Low Flux Reactor (LFR) to test the behaviour of a mixed epithermal neutron and photon field in water phantoms. The neutron and photon fluence distributions have been measured with a BF3 counter and a 235U fission counter and with a GM counter and a Mg(Ar) ionisation chamber, respectively. Absolute determination of the neutron fluence has been performed with foil stacks and of the gamma ray dose with TLD chips. For the design of a treatment planning system for BNCT, the use of a Monte Carlo technique has been investigated. The outcome of these calculations fits the measurements within 1%.