As a result of the experience gained since the issue of Refs. B/Sl and B/S2 it has been found desirable to modify several of the tests hitherto applied to hard composite dielectrics (mouldings and the like) and to introduce a series of “temperature grades” which, although purely arbitrary, are closely related to the different classes of composites in general use.The tests described in this report are not intended as acceptance tests in the purchase of such materials, but it is anticipated that the British Standards Institution will issue a series of purchasing specifications, based largely upon these recommendations, which will define appropriate limits to the specified electrical and mechanical properties for the class of materials covered by this report.The Director of the E.R.A. will welcome comments and suggestions from those who may have occasion to make use of this report.