Lustration in Poland and in the Czech Republic is often interpreted exclusively as a dealing with the memories of the past issues (Vergangenheitbewaeltigung) or a transitory (backward-looking) justice instance. This approach makes unintelligible the endurance of lustration in already consolidated democracies of Central Europe. The article suggests that it is easier to understand it as an instrument of the change of politics rules. Lustration is an attempt to change informal communist rules of secrecy and 'nomenklatura' privileges, which survived the fall of communism. As such lustration aims to overcome a legitimacy crisis inescapable in the situation of evolutionary political regime change. The article defines first the concept of informal rule of politics in order to prove that its endurance makes new democracies of Central Europe a peculiar mix of old and new rules, which is the source of legitimacy vulnerability. Against this theoretical background instances of politics of lustration and decommunization are analyzed (nationalization of communist party property, communist crimes prosecution, lustration proper). The conclusion is that the effectiveness of lustration as a legitimacy enhancement tool is limited. It works only in these fields of politics, where new rules have already prevailed; where a substantial change of rules is needed, lustration fails. Demonstrating the weakness of new democracies to execute laws changing informal rules of communism, lustration makes the legitimacy crisis even more open.