The purpose of this investigation was to develop an apparatus for determining the partition coefficients of the esters used in single well tracer tests. The single well tracer test has been used since 1968 to determine the residual oil saturations in oil reservoirs. Evaluation of the residual oil saturation requires knowledge of the partition coefficient for the chemical tracer used. In this thesis, an experimental apparatus has been developed to determine partition coefficients at reservoir conditions of pressure, temperature, tracer concentration and brine salinity. The apparatus has been designed for pressures up to 5 psi and temperatures up to 2°F. These will cover the range of most oil reservoirs. It consists of a closed loop in which the oil and the brine phases are contacted. Pure tracer is injected in known amounts, and the concentration of tracer in the brine phase is measured while the brine is being continuously circulated. After tracer concentration reaches an equilibrium value, a mass balance calculation gives the concentration of the tracer in the oil phase. Hence, the value of the partition coefficient can be calculated. Another injection of tracer is then made, and the process repeated. In this way the dependence of the partition coefficient on concentration is obtained at a particular temperature and pressure. Preliminary tests were run to determine the partition coefficients of ethyl acetate in synthetic brine (containing 1, ppm sodium chloride) and white oil. The partition coefficient was obtained at 78°F, 12°F and 16°F for tracer concentrations up to .8%. The apparatus developed could be used to determine the partition coefficients for live reservoir oil and samples and formation water.