The production of heavy oil from reservoirs requires much more effort than the production of lighter oils. This is due to the fact that this heavy oil has a significantly higher viscosity; namely, typically a factor 10 to 10000 more viscous than water. This higher viscosity makes the transport through pipes less economically viable due to the enormous pressure drop in the pipe. An interesting way to reduce this pressure drop is by creating core annular flow, which is a two phase flow where the less viscous fluid forms an annulus around the viscous fluid. This significantly reduces the pressure drop in comparison to single phase oil flow. This research focuses on determining the water hold-up in lab experiments of core-annular flow in a horizontal pipe. With the water hold-up and the water cut also the so-called hold-up ratio is known, which is a measure for the oil-water slip. The experiments were conducted for a constant oil flow rate of 0.35 l/s in horizontal pipe with 21 mm diameter. Different water cuts (10 to 20%) and oil/water viscosity ratios (600, 3000) were measured. To determine the water hold-up, the area of the pipe which is occupied by water (or oil) is required. In order to do this the flow is visualised with a high speed camera. Due to the difference in refractive index of the different media that the light has to travel through to eventually reach the lens of the camera, some optical distortion is encountered, which has to be corrected for. The correction procedure was done through a ray tracing analysis which produced a calibration curve that could deduce the actual position of the oil-water interface from the recorded movie. From the temporal and spatial movement of the oil-water interface the waves on this surface were analysed: wave length, wave frequency, wave speed. This was done through applying an autocorrelation function to the interface data. Besides the flow visualisation (to determine the water hold-up and wave characteristics), a pressure transducer was used to measure the pressure drop over a one metre long section of the pipe. The flow visualisation proved that for low water cuts (10%) the hold-up ratio is significantly higher than that for the higher water cuts (15% and 20%). This means that there is relatively more water accumulation and therefore more oil-water slip when the water cut is reduced. The waves on the interface become longer and the frequency becomes lower with an increasing water cut resulting in an almost constant wave speed. To see the effect of a lower viscosity the temperature of the oil was increased from 20 C to 40 C (which decreases the oil-water dynamic viscosity ratio from 3000 to 600). This did not have a noticeable effect on the water hold-up. However, the wave length decreased and the frequency increased which still resulted in a similar wave speed. The results from the experiment have been compared to CFD simulations carried out by PhD candidate Haoyu Li. Interestingly the CFD simulation gives a pressure drop which is roughly 30% lower than the values measured during the experiment. The measured water hold-up fraction is 0.257, whereas the CFD simulation gives 0.255; however, the oil/water interface determined by the simulation is not in agreement with the experimentally recorded interface. The wave frequency given by the CFD simulation is half that which is recorded during the experiment, while the wave length given by the CFD simulation is twice that which is recorded during the experiment. These differences between the CFD simulation and the experiment still lead to an almost equal wave speed.