An HIPR (high internal phase ratio) emulsion of oil in water is prepared by directly mixing 70 to 98% by volume of a viscous oil having a viscosity in the range 200 to 250,000 mPa.s at the mixing temperature with 30 to 2% by volume of an aqueous solution of an emulsifying surfactant or an alkali, percentages being expressed as percentages by volume of the total mixture. Mixing is effected under low shear conditions in the range 10 to 1,000 reciprocal seconds in such manner that an emulsion is formed comprising highly distorted oil droplets having mean droplet diameters in the range 2 to 50 micron separated by thin interfacial films. The emulsions are much less viscous than the oils from which they are prepared and may, optionally after dilution, be pumped through a pipeline. Viscous crude oils may be transported by this method.