Neutralisation efficiency is an important issue in the design of neutral beam injection (NBI) systems for heating and fuelling fusion plasmas. The JET Neutral Beam Injectors system does not reach the efficiency that is expected on the basis of current understanding of the neutralisation process. If this neutralisation efficiency deficit could be eliminated by re-design of the neutralisers, additional neutral beam heating power would become available. The interaction of the high-energy beam ions and the gas in the neutraliser cell results in the partial neutralisation of the beam and also in the formation of a low temperature plasma inside the neutraliser. The interaction between this plasma, the neutral gas and the walls of the cell can give rise to various phenomena capable of depleting the neutral gas molecular density hence reducing the effective neutralisation target. For example, wall pumping, gas heating and modification of the flow regime are all possible effects. This paper presents the results of a number of experiments that were carried out in order to measure the plasma and gas parameters inside the neutraliser. The plasma parameters were obtained using a combination of planar Langmuir probes and a novel electrical diagnostic called a 'plasma eater'. The gas pressure was measured using hot cathode ionisation gauges and capacitance manometers. An attempt has been made to measure the neutral hydrogen gas temperature using a spectroscopic technique that requires the measurement of the intensities of the spectral lines of the hydrogen Fulcher-a system [1]. The experiments were carried out at the JET Neutral Beam Test Bed, which was modified by inserting a flange containing eight ports in place of the isolation gate valve between the first and second stage of the neutraliser neutraliser. The beam was produced from the recently upgraded JET Positive Ion Neutral Injector (PINI) which is capable of delivering a 130 kV/60 A deuterium beam. The measurements were made over a range of beam energies from 40 keV to 90 keV and the results include power, pressure and energy scans in hydrogen.