The possibilities of improving diagnostic methods of oil impregnated paper transformer bushings by the measurement of capacitance C and dielectric dissipation factor tand are investigated in this thesis. The investigation of C and tand were implemented on bushings in service as well as on specially constructed models with content of moisture ranging from 0.1 % to 3.3 % and temperature range from –15 oC to 120 oC. The models were exposed to artificial ageing with the total duration of 3556 hours at 120 oC. It has been concluded that the moisture content has a significant influence on the temperature dependence of C and tand and ageing rate of the insulation. Based on tand at higher temperatures it is possible to distinguish between the moisture content and the insulation ageing. The temperature dependence of tand is saddle-shaped, where the minimum values, with increase of moisture content and the duration of ageing, move toward lower temperatures. Functions have been found that approximate the results of tand measurements in the entire range of the investigated values. The measurement uncertainties of crucial values were estimated, which contributes to the making of reliable diagnostic decisions. New criteria are proposed for diagnostics of bushing condition by capacitance changes, which distinguish normal changes, changes due to grading layers puncture and changes due to excess of moisture and insulation ageing. New criteria for diagnostics of bushing condition by changes in tand in service are proposed as well, for the case when tand of a new bushing is known as well as when it is unknown.