Multiuser interference (MUI) statistical models for impulse radio ultra-wideband (IR-UWB) systems can be an important in providing an accurate estimate of the channel state. As such, it can have a major impact on the overall system performance. In the literature, MUI in the time domain is often approximated with Middleton class A and Gaussian mixture models. We use measurements from an IR-UWB testbed under office Line of Sight (LOS) environments to assess the validity of these models. We also analysis the statistical properties of MUI in the frequency domain. We expect these findings to form the basis for useful interference models.