From theoretical considerations an expression is constructed as a numerical measure of the power of a sheet of a diffusing medium to hide the brightness contrasts of a surface on which it is laid. The dependence of this factor on the transmission and reflection factors of the sheet is exhibited and the effect of varying the thickness of the sheet is discussed. A comparison is made of these theoretical results with published experimental observations. The definition adopted for the hiding power is 50/(residual percentage contrast). The properties of all sheets may be expressed in terms of two constants, of which one is the reflection factor for an infinitely thick sheet, and the other is a factor for converting sheet thicknesses to the proper numerical scale. For a non-absorbing medium the absolute hiding power is 1 + 2xt, and the reflection factor xt/(1 + xt), where t is the thickness of the sheet. For an absorbing medium the absolute hiding power is 1 + λ sinh 2kt + λ (κ - λ) (cosh 2kt - 1) and the reflection factor λ/(κ + coth kt) where κ = cosh 2, λ = sinh 2, and the reflection factor for a very thick sheet is tanh . Expressions are given for the hiding power with respect to any assigned ground contrast. In experimental determinations the measurement of diffuse reflection factors in a photometric sphere is suggested.