Reducing melt borosilicate glass having a UV transmittance of at least 85% for ultraviolet radiation having a wavelength of 254 nm for a sample thickness of 1 mm and a water-resistance of less than 100 micrograms, advantageously less than 62 micrograms, Na.sub.2 O per gram of glass powder according to ISO 719 and a thermal expansion coefficient .alpha..sub.20/300 of from 5 to 6.times.10.sup.-6 K.sup.-1 and comprising from 58 to 65% by weight SiO.sub.2, from 18 to 20.5% by weight B.sub.2 O.sub.3, from 8.1 to 10.4% by weight Al.sub.2O.sub.3, from 0 to 1% by weight CaO, from 0 to 2% by weight BaO, from 0 to 2% by weight SrO, from 0 to 1.5% by weight Li.sub.2 O, from 5.5 to 8.5% by weight Na.sub.2 O, from 0 to 3% by weight K.sub.2 O and from 0 to 2% by weight fluorine. The sum total content of all of the alkali metal oxides present in the glass is not greater than 10% and the sum total content of the CaO, BaO and SrO is not greater than 3% and the molar ratio of Al.sub.2 O.sub.3 : Na.sub.2 O is from 0.6 to 1. The glass is particularly suitable for EPROM windows, UV lamps, photomultipliers, spectrophotometers under difficult climatic conditions and protective tubes for UV lamps immersed in water in waste water treatment operations.