After brief discussion of special vacuum cells developed for this investigation, results are presented showing the effect of adding between 0.1 and 10% moisture content on (a) the dielectric loss tangent and permittivity at frequencies between 50c/s and 100kc/s (b) the dielectric dispersion measured at intervals of 0.1 and 3s and 3 and 300ms (c) the 1 min insulation resistance (d) the 50c/s capacitance increment. In all cases the results were recorded at 10degC intervals between 20°C and 100°C. The effect of the electrode pressure on the electrical properties of paper insulation is discussed and the practical significance of the results and mechanisms giving rise to them are considered.