The motivation for this master’s thesis is building an experimental set-up for measuring the diffusion length of neutrons in water. In the first part, we cover the theoretical basics, define the Boltzmann equation for neutron transport and derive the one-speed diffusion approximation. In the following, we use the OpenMC open source program to simulate particle transport using Monte Carlo methods, preform a series of simulations and use the results to calculate the diffusion lengths of neutrons in water, graphite, concrete, polyethylene and paraffin. At a temperature of 20°C, the calculated diffusion length of neutrons in water is 7,27 cm, in graphite 26,11 cm, in concrete 11,96 cm, in polyethylene 5,02 cm and in paraffin 6,38 cm. In addition, we also check the effect of water temperature on the diffusion length and how the diffusion length values differ when the measurement is made in a stacked pile of material and in plates in water or air. All three methods of determining diffusion lengths are shown to be good. Based on the results obtained, we plan the dimensions of the pool for the experimental part and calculate the expected dose rates at the edges, which amount to 85,8 nSv/h. In the second part, we set up the experimental apparatus. For the pool, we use a water tank with a volume of 1m3 filled with demineralised water. Since we are checking the temperature dependence of the diffusion length, we thermally insulate the tank and install two heaters with combined heating power of 11kW. The water temperature is monitored by five Pt100 temperature sensors and maintained by switching the heaters on and off via a contactor. A submersible pump ensures that the water is mixed and that the temperature is uniform throughout the tank. For the source, we use a Am-Be neutron source and for the detector we use a He-3 gas filled detector. Neutron flux measurements are preformed at nine different distances from the source and at six different temperatures. The measure diffusion length of neutrons in water at 25°C is 7,36 cm, which agrees very well with the calculated one of 7,32 cm. Uncertainties in the measured diffusion length are estimated from uncertainties in the distance of the detector from the source and the water temperature. The relative uncertainties values range from 1% - 2%. A comparison of the measured and calculated results is followed by a conclusion where we present the possibilities of experiment apparatus upgrades and result improvements.