A study was made of electrical breakdown of a liquid (hexachlorodiphenyl) the viscosity of which is very dependent on temperature. It is shown that in uniform fields breakdown results from formation and growth of a vapour bubble in the liquid. This was established by direct microscopic observations of the 'breakdown event' at room temperature and by measurements of times to breakdown as the viscosity was changed by five orders of magnitude between room temperature and 56$\cdot$5$^\circ$C. When the time of voltage application is too short for the vapour bubble to grow to its critical size then the breakdown strength is higher than that obtained under direct voltages. Under 10/50 $\mu$s impulses the breakdown strength of hexachlorodiphenyl at room temperature was 5 MV/cm. It is suggested that vaporization was initiated by development of points of zero pressure in the liquid. Assuming that, in an electric field, development of points of zero pressure resulted from the presence of submicroscopic particle impurities in the liquid, a simple expression was derived for the onset of vaporization. It is shown that this expression gives good predictions for the known dependences of breakdown strength of n-hexane on both temperature and pressure and for the variation of breakdown strength of aliphatic hydrocarbons with molecular weight. It is further shown that the time required for a vapour bubble in n-hexane to grow to the size at which breakdown occurs is comparable with the experimentally measured formative time lag.