The complex Poisson's ratio plays an important role in characterizing the linear dynamic behaviour of solid materials, and occurs in a number of equations used for acoustical and vibration calculus. The ratio of the imaginary part to the real part of complex Poisson's ratio is referred to as Poisson's loss factor. The magnitude of the Poisson's loss factor is investigated in this paper for homogeneous, isotropic, linear solid viscoelastic materials with positive Poisson's ratio. The relation of the Poisson's loss factor to the material damping is determined. It is shown that the magnitude of the Poisson's loss factor is approximately proportional to the difference between the shear and bulk loss factors, and is a rational fractional function of the dynamic Poisson's ratio. In addition, relationships are developed which enable one to determine the approximate magnitude of the Poisson's loss factor from knowledge only of the shear loss factor and the dynamic Poisson's ratio. It is shown that the Poisson's loss factor is smaller than the shear loss factor usually by one order of magnitude at least. Moreover, it is pointed out that the Poisson's loss factor of a high loss and a low loss material may be about the same. Experimental data on two rubbers and a hard plastic are presented to verify the theoretical conclusions.