Multilayer Insulation (MLI) is used for thermal control of spacecraft (S/C). Built-in discontinuities degrade the performance of MLI blankets. In this paper the blanket properties are considered by defining an effective emissivity for the MLI covered S/C surface. The influence of covered area and the influence of number of layers on the effective emissivity is determined by experiments with simple MLI covered test bodies in a space simulation chamber. An equation for the calculation of the effective emissivity of Soviet EVTI-type MLI is presented. The tests are combined with investigations of thermal similarity.