Colin C. Williams and J. Franic found that in Bulgaria, a traditional post-communist country, there is a low association between undeclared work and risk of detection and penalties, but strong association between tax morale and the participation in undeclared work. Their definition of tax morale is based on the mismatch between the formal and informal institutions in the sense of ‘rules of the game’. The contribution suggests that it might be useful to observe the motivation for informal work through the prism of ethics and seeks to develop a tentative new way of measurement of tax morale. For this it adopts an aggregated theoretical model, integrating individual and situational motivational factors and reflects through it on diverse aspects of informality in post-socialist societies. In the S. D. Hunt and S. Vitell model there are constructs accounting for cultural, industry and organizational environments, capturing the specifics that might guide the choice of the optimal degree of formality/informality.