We study the loss rate for a set of lambda/2 coplanar waveguide resonators at millikelvin temperatures (20–900 mK) and different applied powers (3•10−19–10−12 W). The loss rate becomes power independent below a critical power. For a fixed power, the loss rate increases significantly with decreasing temperature. We show that this behavior can be caused by two-level systems in the surrounding dielectric materials. Interestingly, the influence of the two-level systems is of the same order of magnitude for the different material combinations. That leads to the assumption that the nature of these two-level systems is material independent.