Within on site diagnostic tests of power transformers, during the period from 1994 to 2000, about 1260 condenser type bushings equipped with measuring tap were tested. In 1,3 % of the cases the results have shown their insulation system inadequacies. The adaptation procedure of the bushings without measuring tap for on site diagnostics was developed. The procedure was applied to 99 bushings, and the test results indicate that 4 % of the bushings have inadequate insulation system. According to the frequency of unsatisfying insulation status of these bushings, regarding year of manufacturing, it can be concluded that their service life-time is approximately 30 to 35 years. Seven bushing failures was analysed of which four terminated with their explosion and fire of transformer. All four explosions occurred on generator step up transformers of which three were connected with tubular HV connections. According to estimate, the moisture ingresses into insulation system, disconnection of the measuring tap, loose contacts and construction inadequacies are causes of the bushings failures. Connection of the bushings with insufficiently elastic connections may contribute to the moisture ingresses and measuring tap inadequacies. HV testing of tanδ and partial discharge on the bushings taken from service, in the laboratory, it was found that the criteria for their on-site diagnostics in most cases well represent the requirements of standard IEC 60137. The improvement of the criteria for permitted changes of capacitance and tanδ as well as improvement of bushings temperature determination was suggested. Besides C i tanδ bushing testing, other diagnostics methods suitable for bushings in service are described in detail: termovision method, measuring of winding resistance, dissolved gases analysis, partial discharge, bushing oil pressure and bushing visual inspection.