Precipitation of asphaltenes in crude oil has been measured both as a function of pressure and composition. The onset of precipitation has been determined by the measurement of conductivity. A specially designed high pressure conductivity cell has been constructed. Due to the low conductivity of most crude oils the electrode spacing has to be very narrow and the effective electrode area as large as practical. The cell has a coaxial design with inlet and outlet on opposite sides on the cylinder. The cell is rated to 700 bar and 120 deg.C but can be constructed for both higher pressure and temperature. The onset of precipitation is detected by a maximum in the weight normalized conductivity, i.e. the conductivity divided by the weight fraction of oil. By using normal alkanes from ethane to pentane the precipitation onset is shown to increase linearly as a function of n-alkane carbon number. A smaller but significant increase in the onset is found by increasing the hydrostatic pressure. For monophasic live crude the onset is shown by a break in the conductivity curve.