Abstract: The molecular contamination produced by satellite organic materials is an important source of contamination in orbit. It can degradate optical performance and even lead to failure of sensitive surfaces, so its effects must be evaluated. In satellite thermal vacuum tests, optical specimen is used to monitor contamination effects. In this paper, we analyze the experimental data, compare them with those obtained by other authors. It is found that the transmission loss of the optical specimen is greater than nominal values. The possible explanation is that non-uniform contamination layer causes more scatter loss.