It is shown that the impulse strength of oil-impregnated paper, measured on single sheets, is related to the impulse strength of a lapped dielectric using the same paper. Attempts are made to relate the impulse strength to physical characteristics of the paper. General relationships involving apparent density and air impermeability are indicated, but it is shown that, in addition, account needs to be taken of some measurement of uniformity. Means for obtaining such a measurement on small areas of paper are described.